Hi!
The only way to "clean" an engine internaly is to take it apart and inspect it!
The is two main things to look for: bad ballbearings and worn piston/liner.

If the engine has not been flown for several years and previously been run on castor oil fuel ...it definitely need new ballbearings!!
I have seen engines that turn quiet good but where the crankshaft just glided on the innerring of the ballbearing ......the balls being stuck from old castor oil varnish.
Also if the piston is brown /black with lots of carbon build-up from castor oil you have to either get a new piston /liner or clean the piston with fine steelwool (finest grade) or use 3M Scotch-brite pad finest grade. Most of the time if it's a ABC engine this cleaning-up process will take away some of the power the engine had compared to when it was new, but it will still run and throttle well.

Regards!
Jan K
Sweden