You can if you want to do some wheelie or more top speed. But you does not have to change ratio when change engine

You will definitely need a .21 engine mount. You also need a different motor plate or just modify the stock one by enlarge the hole. But we will recommend you buy a new motor plate.

If you plan to use a rear exhaust engine, you will need a U shape manifold because exhaust will be existing at fuel tank. And you also need a U shape middle manifold to be able to ues the exhaust at the same location. Or you can just have the exhausting shooting toward the back without using the middle manifold.

http://www.radiocontrolzone.com/foru...34#post1032834

Here is a picture. Thats the setup you will need to run a .21. Doesnt have to be exact but thats the idea.

.21 class engine recommendaion: RB, Orion, OS, Sirio, or Novarossi are all excellent engines. You will be definitely happy with their engines and No way you can go wrong with those.