Not that I have any solution, but I'll tell you of my experiment for an easy to fly ( ~70 mph on 60' lines) " Sport Racing" event. As many of you may have read, the USA & Canada have an entry level sport racing class that uses the stock Fox .35 stunt motor. There are local variations regarding model designs, but they are all very similar with only detail variances. Basically a built up 300 sq. inch wing profile model. Record speed is 85 MPH.
With the demise of the Fox Mfg company, the stock O.S. 25LA motor is offered as an alternative for those wishing to get away from the (vibrating) Foxes. Inasmuch as my own Fox .35 (Mongoose) model consistently races very well at speeds just short of the record... I agree the O.S. is even easier to hand start & run and is easier to go faster with than the venerable old Fox. In the American Pacific Northwest, interest in the 4 remaining classes ( Mouse, Clown, Fox race, & NWSS) of racing have dwindled to less than 3 entries per class. You can't fail to place over here as long as you enter. Note: NWSR & NWSuperSport race are classes that use obsolete motors ( Fox 35, O.S. 25 & K&B 40).
 
So I thought perhaps a RACING INCENTIVE might be worth a try.
I tried one of these O.S. 25 engines on an old profile sport racer complete with muffler for fun over grass, as I was teaching my (60+ yr old) wife how to use a shut-off. Well darned if that model didn't start 1 -2 flips every time hot or cold at perhaps 10 -15 MPH slower than normal. My neophyte wife Dianne really liked the control the shut off gave her.

So I made it well known in advance that anybody using a stock muffled motor (either Fox or O.S.) in this existing NWSR class at the big NW Regionals event would receive a $100.00 cash prize, for the best heat time. You didn't have to win, just have the best heat time with a muffled model. I didn't want to burden the CD with a new class, so this fit in nicely with the existing NWSR (North West Sport Race) event.

On the big contest day with perfect weather all THREE of us showed up. Nobody was a new racer...
This incentive attracted absolutely zero interest in new entries. One local racer actually built a new model with a muffled O.S. engine & ended up winning the event due to flawless pit work over the faster unmuffled Fox in second place. My model placed third a few seconds behind...
This race proved the O.S. was super easy to work with, & very quiet with the muffler. One spectator really liked the reduced speed (~65 MPH) and said he might even have to try it at these speeds...

So I learned that I can't even BUY interest in C/L racing over here.
It was a failed experiment that cost me $100. Ah well, if you don't try you'll never know I suppose. Wish I had a story with a better ending...

Paul G.