I have put both in. The 18 cvr (S) will require some clearancing on the motor mount and the exhaust header will bolt up in same location. The motor mount has some clearance issues with the pull start. The carb will hit the transmission just slightly - grind away a small section of the gear case and it will fit in.

I have also installed the TZ. Direct fit because the pull start is smaller but you will need a new exhaust header. The exhaust exits the rear instead of the side of the engine. The TZ is one bad dude - I love mine. I do not believe the TZ is available with the short shaft (S) but it is no big deal - you just need to cut a few threads off the shaft to make it short.

I have a post called "2 speed transmission in a rustler". The video attached shows both motors - the cvr and the TZ being installed.

Good luck and remember anything is possible if you put your mind to it.