yes, a XR200 top end will bolt onto a 200X bottom end, as long as the XR200 top end is not the duel carb, 4 valve head. the XR200 does have a larger bore (65.5mm xr200 vs 65mm 200x) and a higher compression ratio (10:1 xr200 vs 9.6:1 200x). i would also recommend the XR200 carb, intake manifold, and cam, as these are performance upgrades for a 200X.
My Rides
1983 ATC200 - U-Haul edition - great mudder - SOLD
1984 ATC200X - sucked a valve - frame up resto & engine rebuild with performance parts in the works
1987 Suzuki DR200
199? Go-ped Sport