Cam chain and sprockets

I am restoring a 1978 KZ650B with 4K miles. It had been sitting up for 30 years and the engine was frozen. I have it all free now and was adjusting the valves with the shims when I noticed that the cam chain doesn't seem to fit freely on the sprockets. The sprockets have no wear that I can tell and it seems that the chain has stretched or was defective from the beginning. Like I said, the bike only has 4k miles on it and the exhaust valves were bent from it apparently jumping time. Is the cam chain my problem? The rest of the engine (after replacing the bent valves) appears to have no wear and damage that I can see. I did remove the oil pan and checked everything from the bottom end.

My question is the chain stretched as it would seem to be as it rides high on the sprockets unless forced down and can the chain be replaced with one with a master link so I don't have to pull the crankshaft out? I would hate to have to tear down the cylinders and split the crankcase unless totally necessary. HELP!