That same rotostart backplate fits a lot of other engines, including Ofna Force and the Picco engines. It is not limited to .21 either. It will fit engines like the Force .26 and Picco .27, so don't limit yourself to .21 unless it is for racing restriction reasons.

Also, what are you going to do with the stock 24.7? If you are going to put it on another vehicle, you're gonna need a starter for it too. I have a brand new Mammoth ST and I'm dropping in a Werks Racing Collari .32MT. The stock 24.7 in being transplanted into my Ofna Blazer SST.

For you, I would look into .26 or larger. Not all of them are tuned the same. It all depends on what you like. Ssome are speed demons that can pull high RPM, while others are stump pullers that product prodigious amounts of low and mid torque. The SH .28 is not a bad choice if you're on a tight budget. Hobby People usually has it on sale for less than $150. You might have to tweak the gearing ratio if the new engine powerband is very different from the stock 24.7, but it's not a requirement if you are just bashing.