Norm's right. Any XR 185, XR 200 (pre-R models), XR200R 1982-1983, XR200R 1986-current.
In fact, an old XL 125 split head model will also fit - won't have much power, but it'll bolt right in.
The XR 185, and early XR200's had weaker kicker gears and skinnier 3rd and 4th gears. The newer 1986-current models are a little stronger. Same horsepower tho.