Its a 1974.
Its a significantly different motor to the YZ-A and YZ-B (although they are clearly related).
FWIW, the DT-A motor is interchangable with the MX-B and YZ-C.
As Jim said, you want an RT1 motor - at least it ought to bolt into the YZ frame. But don't go thinking that an RT1 motor is all that similar to a YZ360 motor. Adam (YZ360A) has put up some good posts about the differences between the two.
I've got some spare reed valve RT1 top ends, if that helps you at all. Quicky had a rough RT1 motor (and incomplete frame) that he'd been trying to get rid of for $SFA.
Generally speaking, they're not too hard to find, although good 'uns seem to be thin on the ground.
And good luck with all of the "I want to buy the YZ" PMs you're going to get...