EDIT: I assume you're talking about 5 speed parts below. While the M20's 4hp22 would bolt to the M50, it would be a fun time getting it all to work.
Yes and yes. All of the 5 speed parts (driveshaft, shift console) may bolt right in, too -- I don't know the relative difference between the M20 and M50 mounting points, if any.
The question is, what is there to do with the rest of the car?
best, whit