I wish we would get back on thread...
It used to be very common to swap cylinders from one model to another to take advantage of better porting in a different type of engine. Most cylinders were a square 4 bolt pattern and tended to be on the even mm size so fits were quite common. I would guess if you scoured the MX parts bins you'd find several cylinders that would fit. There are several caveats, unless you want to do some welding and machining:
1) it would have to be a cylinder induction jug,
2) it would likely need to be aircooled
This would pretty much limit us to MX engines earlier than 1985 although there are still a lot of high output aircooled snowmobiles made.
Watercooled MX engines after 1980 pretty much all came with internal case to cyinder waterpassages, case induction and powervalve.
The radiator is a lot of extra complication and about 5-10lbs more weight. My son has run a DT200 cylinder in his Blaster and I can attest that we are making more hp with modified Blaster aircooled cyinders.
Steve