Don't know if it will bolt up or not, but the 250R motor, as with most ATV motors, has a counterbalancer shaft to smooth vibration. The CR motor does not. Why not try putting a CRF450 motor in it??
[FONT="Palatino Linotype"]~My Rides (to name a few) ~
1986 ATC250R, 1985 ATC350X
1985 ATC250R Missile (90% done)
1983 ATC185S
1999 TRX416EX
2003 Honda CBR600RR
1990 Yamaha Zuma
1985 XR80 w/100 motor
1967 Honda CB77, '65 CL77
1962 Puch 250 SGS
1984 XR350R (owned since I was a kid)
1972 Suzuki T350 Rebel, '73 T500 Titan
1964 BSA A65 (needs restored)
1968 BMW R60/2 mint original bike
1976 Kawasaki KH500 triple
Anybody have parts to trade for the above models?