When you say “spin over” do you mean the engine rotated or did it actually run ?
How are you trying to rotate the engine now? Kick start? If the engine isn’t running how do you expect it to go into gear ? One of the shafts must be turning in order to shift into gear
USUALLY so that the dogs line up.
Have you checked the oil level in the trans ? Did you ever dump any oil down the plug hole?
What is your local climate like? Humid or dry? Stored inside or outside? Covered ?
As what to do at this point, first power wash it ! Pull the exhaust and see what you see. Depending on where the piston is you might not see much. Pull the cylinder head next. If the cylinder is rusty apply penetrating oil to piston/cylinder while working on other things.
Next I would pull the clutch cover and inspect everything. Could be kicker idler seized, clutch basket seized, heavy rust on the gears.
At this point only you can figure it out.
So many things could be causing it, or even a combination of things.
Good luck and let us know what you find .