Most horizontal M-18 equipped cubs had the oil filter adaptor on the block for divorced /remote filters using oil lines.
So If a person wanted to use a filter on the block,all one had to do was use the applicable adaptor.(and trim the cooling tins for block mounting)
You already have the block mounted filter.
currently I have 3 engines with block mounted filters and one with a remote filter.
That crank on epay looks usable,and from the picture I don't see any problem with the blemish on one rod journal.
That said, without putting a mike on the rod journals, one can't see the size or any taper.
But usually either they are quite obviously damaged, or usable.
I think your closure plate will work as it apperars to have the PTO clutch mounting holes,and the internal oil passages are in the block and are all the same.
Some of the replacement supplied engines had different length crank to fit say a irragation pump or a tapered crank to fit a generator application, just to name a few,
I've seem one on a bandsaw in a saw mill utilizing 4 drive belts.
I'd say you just need to change your crank to enable you to mount a PTO clutch on your engine.
And if you did have to change the closure plate, its just a bolt on cover.
I'll not get into making a Vertical engine long crankshaft fit into a horizontal engine but it can be done, but it needs shortened and some lathe work, and a keyway milled, but its an acceptable job for us that like to tinker
I'm guessing your red replacement engine possibly came with a longer crankshaft for say a pulley or irragation application, and somewhere along the line it got broken and was transplanted into your tractor just using for motive power, not PTO use. but its just a guess.
Or theory #2:
"Bubba"
bottomed out the PTO retaining bolt in the center of the crankshaft with an impact wrench and snapped the end of the crankshaft off.