Regular 3 in 1 oil has quite good anti rust properties. You can squirt a whole lot in through the tiny holes, then block 'em up and rotate the bike around until you are sure every part has been coated, then let it dribble out.
Other coating liquids, like Waxoyl, are too thick for this, even if you heat them and the bike up. They are liable to just block the tube and not get to the corners. And IMHO, have no better rust deterring properties than 3:1 oil.
There are things called 'rust converters' based on phosphoric acid, but I don't like using these on places I can't see. Regular dosing with 3:1 seems to be cheap and effective.
None of this will reverse any damage that has been done. Succumbing to 'metal moth' is a one way street. We can only hope to slow it down.