Actually if you see what appears to be rust in the bearing area, do NOT assume you have to replace them.

I find that a good healthy amount of "Dawn Power Dissolver" squirted into the engine works wonders.

Remove the carb, muffler, glow plug and backplate and squirt Dissolver into the engine

Let it sit for about 8 minutes then flush the engine with water.

Repeat, and take a small epoxy brush with some more Dissolver to try to loosen any remaining reddish areas you may see inside the engine.

Flush again with hot water, shake out all the water you can, then flush the water out with WD40, followed by glow fuel.

Then turn the engine over without the plug in place.

Then if you feel any "grittiness" replace the bearings. If the engine seems to turn over very smoothly without any hint of grittiness, crank it over with a starter for a bit and check again. If it still feels good, then put it back together and start it up.

I've now taken well over 10 engines that people have tossed out, and run them through the above.

They have all been flying nicely, donated to club trainers and newbies just getting into the hobby.
I just did an old OS FP60 this way three weeks ago and I already have 8 flights on the trainer it now sits in.