Some time ago I tried to repair one with no result, my advice get a new one even an unidentical one and adapt it.
I have two Royal 20's (one of my favorite sets), and the tuning cap in them is smaller than most
other Transistor sets. Not only that, but the shaft is somewhat unique.
I would suggest just being on the lookout for another Royal 20 on eBay, and combining the best of each together. They don't go for that much.
Recapping is somewhat of a PITA, but they'll work very well on a few button cells, using a screw and bolt as an adjustable spacer in the battery compartment.