I have been looking in to doing this as well. apparently the person who knows most about this is jenndnn3. search for a thread called "xr200 transmission in a 200x" that one has the best info I have found yet. on page 2 of that post, jenn chimes in with a link to info on .org.
sounds like everything is interchangeable but the 200x kick gear is too wide for the xr200 counter gear. but you need the other 200x parts to get the forward kick.
still have not found anything definitive on exactly what years work. sounds like 83-85. seems you need trans from before they went to twin exhaust. dont know why that makes a difference but that was what I read somewhere.
Trikes: All Honda: 84 250r, 85 86 and 87 250sx, 84 and 85 125m, 79 82 84 and 85 110's, 73 us 90 and 77 atc90, 75 atc70
Cycles: Honda: 71 cb100, 78 cb750 coming soon cb750 trike. Yamaha: 67 yds 3 (250cc, 2cyl, 2 stroke), 82 750 SECA
Other toys: Chenoweth VW sandrail dune buggy, old race go kart, racing mower, 76 Arctic Cat Pantera 440 snowmobile
KEEP IT ON ALL 3 OR AT LEAST 2 AT A TIME