the .50 is ring..so it's not going to have a tight comprassion like ABC tapered cylinder
i wouldn't trun the engine anymore.
if there's rust on the shaft counter wiegth...most likey there's gunk build up
on the ball bearings..you need to break that up and flush it
to prevent scratched to the piston/sleeving and a possible leak to the front bearnings
in the future.
just take it apart and soak let it soak over night and flush it.
grease remover from $.99 store works for me.

it's only a single cylinder with 3 moving parts.
you took the back plate off already..
just take the head off..the sleeving should slide right out..
if not lube it as much as possiable. if needed,heat it to expand the casing
or use the back end of a tooth brush to push up the sleeving.

the rod disconnect from the shaft. take the piston/rod out.
the shaft should slide right out

lube the o ring on the carb to prevent it from drying or cracking

use blue loctite (not perminate) to prevent the screws from coming loose

seal the backplate, carb with rtv or something to prevent air leaks.

use after run oil.lol