Parts are easily and often swapped on these bikes. It could just be the tensioner but it might be the whole cylinder block or even the whole motor. You need to investigate because it could come into play if you need motor parts.
There were basically two versions of the 650 motor. The ones up to '73 are called a "256" motor. '74 and newer are called a "447" motor. Many internal parts differ, things like pistons & rings, cam, cam chain, and rods.