Ok,
So to reiterate, I pointed out David Kirks "Killer Kohler build". I wanted you to read through it so you can see how far he went to have an outstanding "stock" motor.
While I think David did a great job, I don't think all he did was necessary. That said, I will explain. When building a true "hot rod" motor (whatever application, racing, pulling, ect) there are a lot of things you can do to gain a little HP, here and there. One small thing done doesn't necessarily make a big difference, but done in conjunction with several small things they start to add up. Again, David did a great job..... but as most car "hot rodders" do when they move into a tractor world, is they fail to build according to application. They carry over things they do to high horsepower, high RPM motors thinking that every engine needs those things to be "great". Well.... they don't. Practical application is everything..... building a motor for a pulling tractor IS NOT the same as building a race car engine. Building a stock class pulling engine for a GT IS NOT the same as building a good running plow tractor engine. A plow tractor has to be expected to perform for hours on end, under load, and varying loads. Not really something that happens on a pulling or race track. Tractor pulling engines run 300' and get shut off. Done. Maybe started back up and pull another 300' then they get shut off, and so on. I think you get the picture.
All that said, here are some things I have found with David's build that hold a lot of weight and make more difference than you would think.
Before I start on that though, just a few overhaul tips. I'll explain some more details below, but just to get you started, you don't need to spend a hunk of money on Kohler parts. I get all my overhaul kits from ebay. I buy off one buyer in particular.
Here is a link to his ebay store. I already linked you up to the Kohler section on his page. I don't order off ebay, I call him directly. His number and business hours are listed. I just tell him what I want, give him my card number and I get it in a couple days. Get the most inclusive overhaul kit for the K301. Valves, tune up, the hole shebang. Order new valve guides too. For another like $35 it's worth it. Kevin is a good guy, and I've had no issues whatsoever with aftermarket parts. (From him, or any of the other hundred(s) of motors I've built.)
Bore it. Don't skip that. Have the machine shop tell you what size you are going to need BEFORE ordering the kit.
Grind the crank. Only reason I wouldn't do this is if the crank shops says you don't need to. Most always they need ground. Again, do this BEFORE placing an order for the overhaul kit.
*IF* the engine still has balance gears, remove them. Throw them over your shoulder and don't ever touch them again, unless it's to move them to the scrap pile. They aren't worth putting back in, and they can grenade leaving all the hard work for naught.
Otherwise, standard rebuilt stuff. I'll cover the "fun" stuff,,,,,,, now.
Valvetrain:
*David talked about using a file on the edge of the valve and adding a radius to the valve reliefs. It does help. Radius them like he suggested. It will make the gas flow better.
*Port/polishing: Do it. It helps. Do the intake, and exhaust. You don't have to "hog" them out, just try and get them as smooth as you can. I use a Dremel with the handheld wand to do it. It's easier to do with the valve guides removed. I also suggest polishing the cast threaded adapter to the muffler. (Unless you go to a header/stack pipe with a MWSC adapter. More on that later.)
David talk about working the intake port lead in where the carb mounts. This is a good thing and he is correct that the factory design can be improved. I would tear the carb down and install it with the butterfly out and look in the hole. See if you can do some grinding to improve it, then let it go. If you want to take the time to fill it with JB and make a smooth transition, then go ahead. I think it will make a difference, but as of yet, I haven't done it.
*Do a 3 angle valve job. David stated he didn't. If you can't do that yourself, then have the shop boring it to do the valves. If they argue, tell them to do what you want. I had one shop tell me it wasn't necessary. I told them I didn't ask for help on how to build it, only asked them to do the work I requested. (Usually I do it myself, but my valve equipment was down.)
*While we are on valve train, spend the extra bucks and put in all new guides. I ALWAYS replace valves on an overhaul. Going to grind the new ones anyway, so doing the 3 angle is just part of the new deal.
Piston:
*Mahle pistons are great, but you aren't building a pulling motor or a hot rod. Whatever piston comes in the overhaul kit will be fine. If you want to pay big bucks for a Mahle piston, or Kohler one, that's up to you, but I don't think it's necessary. For a plow tractor, it will be just fine.
Block:
*Doing that deburring David talks about in the valve area on the cooling fins for improved cooling flow.... that's a good idea. I haven't done that in the past, but I think I will on my next custom build. Can't hurt a thing, and my Dremel will be faster than David's hacksaw blade and file, lol.
*I check the deck surface with my glass sheet I use to plane heads. I simply set it on top of the deck and work it to see if the deck starts to polish flat. If it doesn't I keep going until it does. There are faster ways of doing this, but they cost money if you are paying for them. You don't need to take off a lot of metal, but you will see the pulled metal around the head bolt holes. You can file it down with something bigger, then go ahead with the glass until yo use that it is going to be flat. If you notice a large area that isn't flat, look into getting the block decked. Machine shop can do that, but don't take off any more than necessary, and make sure they tell you how much they took off. Never seen a Kohler need any more than just some sanding to the deck with the glass sheet.... I'm just making a general statement on what to do if you encounter it.
*Continued due to over size limit for a post*