I have never tried it, so somebody else might chime in on it, but your engine case might be salvageable. Since the hole in it is only "about the size of a dime", you can probably patch it with JB weld, or maybe drill it out to perfectly round and plug it. Your crankshaft might be fine, or maybe just need a re-grind on the rod and main journals. The rest can be had on the used market, if you can't find them new. I'm betting you can rebuild it for $3-400, rather than spending upwards of $1200 for an engine somebody else rebuilt, or $2000 for a new one. Then again, if I could afford it, I'd be re-powering with one of the newer OHV V-twin engines.
Since I have a KT17 Series 1 in my 682, I'm going to start trying to cross-reference parts and see whether I can use parts from other engines to replace NLA parts. Right now it's running fine, but eventually I'll need to overhaul it. I'm thinking that parts, such as rods, pistons, heads, cylinders, and valves are probably pretty similar between the various iterations of the Kohler flathead twins and might be adaptable. Might even be able to use some B&S parts.
Try this link, somebody posted a while back. Might be a place to start:
http://kustomsmallengine.com/main.sc