Both belts are still intact? I would pull the valve covers (four bolts each side) and at least look for evidence of damage there; broken cam, rockers, bent valves. If it’s not moving either direction, this is less likely.
Lots of guys have luck spraying some penetrant in the plug holes and rocking the back wheel with it in gear. Heat can help too. (Acetone/ATF is a common suggestion/ keep it off paint) Leave it a day, rock it, repeat. If it’s just a little corrosion you probably get away with it. If it’s a lot, you might make it worse.
Safest way, but expensive is pull the head(s). If it’s been sitting a while, the original head gaskets are apt to have a short lifespan anyways.
Whatever I suggest here should be given ample time for a moderator to delicately correct. I apologize in advance.
77 WING, 1200 engine with 77 heads, cams, gl1100 foot pegs, Magna V65 front end, 764A carbs, [-gone Suzuki M109 monoshock--, replaced with gl1100 shocks] gl 1200 swing arm, gl1500 final drive, wheel and rear brakes Valkyrie seat, Meanstreak tank, Sportster pipes, Power Arc ignition off crank.
77 Wing. black
83 Wing, in pieces
"Continuing education is important even if the subject matter is fairly useless (as in this case)."---Greg Foresi