XRAY - Model racing cars

Two engines replaces the Toki engines. P-0 and Drake. Picco P-0, IMO the better of the two. May have use a dremel to custom fit. Take your time, you'll be satisfied of performance.
As for pinion you choose, you have experience with 8th & 10th scale model. You know what you want, low or Hi torque. Its easy for you to decide.

For Exhaust system, Stock is ok, but better of useing the NT18T 3 piece exhaust system. Like 8th & 10th scale you have tuned pipes. Nt18T 3 piece more adjustable on the rubber coupler. The shorter between header & muffler you have low end torque, Longer the coupler the higher torque you get.

Have fun with it. The little bugger will run like the little monster.