YZ125 until 1980.
DT125 (all).
DT175 (all).
DT100 (all, I think. Definitely the ones with the cross-over pipe).
IT125 (all).
IT175 until 1980.
YT125/175 engines might fit, but pull start and auto clutch and stuff means you probably don't want one anyhow.
Watch out for the LT1/AT1/CT1/AG motors with the very different lower engine mounting point - it hangs a lot lower and stands out like dog's balls if you know to look for it.
The later YZ125 and IT175s have the chain on the right.
The later-still YZ125s (1986 and later) and DT200s are a different motor again, and don't fit.
Not sure about the DT200Ns (single radiator, red shroud), but I doubt they'll bolt in - and they're water cooled anyhow.