Not sure which engine you have, but if its a V4 Yamaha yep its over powered. The good news is that changing the engine is quite easy. Perhaps a 75 - 90 2 stroke which will be light would be better. The Yam 90 is a gem.
As for the tubes, your right painting them with a brush leaves brush marks in the paint surface. Also no one will buy it if its been painted because they can't see what condition the tubes arre underneath. You could cantact Avon in South Wales to see how much a new set of tubes are. Then when you have picked yourself up off the floor ask Henshaws or Eurocraft for a more realistic quote. Alternatively Avon use good quality materials so if its only cosmetic you have nothing to loose re painting them with flexi paint again trying to get a better finish.