Are you sure you were using Stunt .35's on Goodyear? No throttle as that is supposed to be for control line. They are still popular for stunt, at least for those who like to fly stunt but don't really compete and prefer not to spend as much on the more expensive larger stunt planes. So you should be able to sell your Fox's. You won't get that much for them unless really old or one of the odd varients that were not around very long.

These engines are made for torque spinning a 10-6 at around 10 to 11K and with a 4-2-4 break right out of the box. Easy to start and reliable. The only real problems is that they vibrate a bit, and the stock needle will break off on a profile on the first inverted landing or hard landing (aka crash).

As far as new designs, the designs are far superior to most. The only designs they need are for larger engines.