If it ain't rattling very bad (with the clutch disengaged) and it ain't smoking, just put some fresh belts on and check the valve clearance. You'll most likely have plenty of life left. Check for snapped head studs also (oil leakage at head and the nuts will spin freely.) If they haven't broken by now they've either been replaced or are gonna be fine. Just don't wind it up over 7500 if it's the original studs. No need to anyway. It's a Ducati, and a tuned for grunt one at that. You're gonna love riding it.
'81 900 SD Darmah