you can use a new engine and just put your old headers on it to take care of the ECM deal
all of the motor mounts match up across all years of XBs, the heads, jugs, and back end of the cases externally never changed
if you got a new model engine you would also need to get the oil lines for it, but the swing-arm is the same so you can just buy the oil lines themselves and put them on without doing anything custom
The fuel and ignition systems changes don't affect the motor itself really, you can run the old ECM and sensors on a new engine just fine. But there is only 1 O2 sensor on the old models so simply swapping headers fixes that problem. Everything else is a direct swap over.
any specific questions about the differences in the models besides the general one?
there are a lot of extra XB engines that show up on Ebay and Buellsterparts