Ok this is from the memory of an old fart:
I have no experience with mag 20's only mag 18's.
On the m-18, I needed a good crankshaft, so I used the vertical one as my horiz was junk.
I had to cut it off and machine (lathe work) it to 1-1/8" dia. and make a short bushing on the 1" undersize section @ the end of the clutch bearing area.
Then drill/tap the end of the crank for the pto clutch retaining bolt. as it was a vertical and much too long, and with it shortened, the thread was gone .
Also a captive keyway had to be milled in the crank for the key that drives the pto clutch.
IIRR the M-20"s are 1-1/4" diameter not sure as I have said,I have no knowlege of them so that might not work without measuring and doing figuring.
M-18's are 1-1/8 so this m-20 crank machining might be very different or not be feasible at all.
The block was not drilled for a dipstick tube, so I had to set it up on my B'port mill and drill it @ a compound angle with a special 16" drill.
Can't remember the size but it was over 5/16, maybe 3/8 Dia.
Then drill/tap the dipstick tube retaining clip.
Then I had to drill the oil passage that was not drilled in the block where the suction tube mounts for the oil screen in the bottom of the sump
( again Bridgeport work)
Also the cup plug had to be pressed into the block, to block off the new drilled oil passage hole @ the pto cover mating area, to duplicate the factory machining.
There are 2 different diameters on that modification, to contend with.
Then there are 2 gravity oil drain back- to- sump holes, not on horizontal blocks
(one in each cylinder) that are drilled on vertical blocks that have to be plugged,---- if you don't plug them, it will flood the valve/spring chamber and it will consume huge quantities of oil.
I turned up aluminum plugs for this, with an interference fit and pressed them in the holes and headed then, so they would stay put and be retained by the cylinder spigot when assembled.
You will have to use your horz. oil screen pickup box,
as well as your pto end cover.
Prolly more but I can't remember just now.
Like I said in the old post referenced to,
it ain't a bolt together and go.
And without having done a mag 20, all bets are off.
This should answer your personal message as well as answer interested others.
I did it as a challenge and the engine is running well today, @ least 800 hrs later.
