The best answer I can give is to take all kinds of measurements. You need to measure how long the "engine cradle" compartment is. You need to measure how high. You would need to know how wide each of the engine bolting areas are. Then take your measurements to a local salvage yard and start looking. It may be possible to "stretch" the frame, or to make your own engine bolt mounting areas if some engine you find does not fit exact.
My best guess would be that a CB550 or maybe a 650 would fit. I would not go any bigger than the 650 size engine. There should be plenty of them around from the early 80's But I will not guarantee it, as I have never swapped out a CB500 engine before.
