ORIGINAL: Sport_Pilot
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.
The Fox Stunt is a good engine for low-performance "Slow" competition classes. Everyone uses the same engine, so it's competitive by definition. Sometimes the rules specify the airframe as well. Racing classes like this abound, and are regional or club-based, examples include Foxy Hazel Racing, NW Sport Race, the Skyray 35 Race they hold at the SIG contest every year. Performance may be "low", but you can still get Fox Stunt powered racers up to 100mph with the proper prop and airframe. Similar were the Fox 35 combat competitions we used to have in the 80s and 90s - this was before Speed Limit Combat appeared. Typically 60-65mph on 60 foot lines - made for some really leisurely combat - you could actually THINK about what you were going to do next. For all the talk about 10x6 props, we found that a 8x6 worked really well on a 35 Stunt in combat - you'd get maybe 68 mph.
I fixed the needle problem with - how else? A bladder and remote needle. All my engines other than 1/2-As and F2Ds are set up this way.
Iskandar