redcat tends to run SH engines and SH usually run a pilot shaft crank, traxxas runs standard style cranks, flywheel and flywheel nut might need to be swapped.This is the 15 pro: https://www.amainhobbies.com/traxxas-trx-pro-.15-pullstart-engine-w-std-crank-tra4010/p18970
This is the trx 2.5:
and the 2.5R:
All three are .15 engines. The 15 pro is a side exhaust/rotary carb/standard shaft crank engine.
The 2.5/2.5R are traxxas proprietary sized rear exhaust, slide carb, typically pilot shaft crank.
From what I've seen of redcat, many of the 1/10th run engines similar to the first, but I don't know if the specific one your asking about uses a slide carb or rotary carb.