The CL3B is a really good engine, and frugal on fuel. It may not be as stupidly fast as a C6 or something, but when I had mine, it was as quick down the straight and poky out of the corners as most other engines. Also give the CL7B (non-STI) a look. Think CL3B but with more power, it's also not really expensive and has been really easy to tune (my brother has one in his truggy - an TRF801XT - so it'll definately fit too!).
Other than those, an RB K9 as Mallet said and I mentioned earlier.
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