So just a quick update I have discovered the valve timing is out and removing the cover and checking alignments of the timing dots it's out ! T mark on rotor equals dots on exhaust and valve cam not aligning with the mating surface !
If it's out less than 1 tooth on the camshaft sprocket, that would be the stretch in the camshaft chain + the stretch in the secondary drive chain.
If it's out more than 1 tooth, then you have some investigating to do to find out why the camshaft chain got enough slack in it to allow the timing to jump.
Things to check-
Cam chain tensioning roller - chunks have been known to break off. Remember they are rubber immersed in oil and now 40 years old. It would be on my list of things to replace when acquiring a used bike regardless of the mileage on the bike at the time of purchase.
Cam chain tensioner- Known to have a weak locking mechanism on the spring loaded plunger. There are 3 possible fixes. Replace with a ZX11 tensioner, or replace with a manually adjusted tensioner, or modify the stock tensioner to fit a locking bolt on the end of the tensioner. You can find info on all of these using the search tab and setting the time span to "any date"
Last but not least, once you have the timing corrected, you need to do either a leakdown test (preferred) or a compression test to determine if you bent a valve. These are interference engines and skipping a tooth on the camshaft usually bends a valve(s).
Keep us in the loop and I hope all turns out well.
KB