If it had the cummins stock, then it would definitely fit. It is a common motor in RE buses, so it really shouldn't be a problem. If it had something else in there before, it might get tricky. But I bet you could find one with a blown 8.3. Finding a bus with a blown motor AND tranny might be a bit of a unicorn. Swapping in the close ratio gearbox is going to ruin your performance and be a MAJOR bummer--You'll be missing out on all the fun of that 8.3. You'd be better off with a stock MT643. IMHO if the bus has a working MT643 or MD3060, only swap the engine--it will be easier, cheaper, and perform better while being essentially the same trans as the MT653CR in terms of the maintenance required from you. If it has the AT545, I'd swap the trans. But the AT545 is not very common in RE buses, and it would never appear in one that has the 8.3 from the factory.
Patina enthusiast and professional busman