I have an old rc engine and I was wondering if I replace the crankshaft, do I also need to replace the bearings as well?
And the other way around: If I replace the bearings, do I also need to replace the crankshaft?

New crankshaft - replace the bearings. Good insurance against failure. Replace bearings - only replace the crankshaft if it’s scored up or shows evidence of rubbing on the crankcase or dirt ingestion.