Well, after disassembling a known good differential, all of the gears in the bad one have at least some wear and aren't work reusing. I have found, what I believe is, a NIP differential for a Robbe Panther in Germany. It is hard to visually inspect it based on the internet pics, but by description, it should be correct. So, now have to wait for that to come.
So, with a new differential (hopefully), I think I have enough parts to build two rigs, one nitro and one electric.
From what I can see, the stock OS 21 is a FSR-B, and from an article I read, it was rated at 1hp @ 24000 RPM. So I am just guessing here, but I am going to shoot for a drive set up that will give me the same driveshaft RPM as the stock assembly created with a 24K RPM engine. The stock P/S was 12/50 on the OS, and the stock Arrma is 18/46. Can someone please check my math attached and make sure I am not doing something stupid? The plan is just to run 2S lipo, and 2000kv motors are relatively common. There is a barely used 2200kv CC setup locally I may get just to save some money. I don't think I can go smaller than the 18t pinion with the 46t spur in the stock mount, but maybe I can just limit top speed through programming.
I build RCs like people would have done back in the '90s ..................................... if they had 3D printers.