Well, it depends on whether you need one first- the engine you have in there already, if it works, should be perfectly sufficient. If you've never experienced nitro before then you'll find that the car and engine you've got will be much much faster than any RC vehicle you've ever seen or used before. Buying a new engine is only really necessary in my opinion if the current one is broken, or you're serious about competition and need that performance edge over the competitors. In you're current situation I'd recommend sticking with what you've got if it works