Hi Mark
Welcome to the site.
Your 550 is without doubt worth rebuilding and using again.
LAB is bang on when he says the 500 four is more highly regarded as a classic machine than the 550. However when new the 550 was regarded as more refined and smooth than the 500 as well as handling better.
I had a 550K3 (four tail pipes) in the late 70's. It needed a new cam chain and tensioner at 15000 miles and a gearbox rebuild at 19000 miles because of jumping out of third gear. Both common faults, the 3rd gear fault usualy on hard used and well thrashed machines, as mine was (teenager you see).
I would check both items whilst your motor is down!
Once its built use and enjoy, just change the oil every 1000 miles(I didn't) and the motor will run for ever.
Grizzly