I am sure NO ONE will argue with me about the following opinion.
(1) Sell the RS4 and get the R40 (or buy the Japanese version R40 2004 spec with almost all the necessary options standard in the package - About $385 shipped from HK or Japan) you cannot convert a RS4 to R40, period....car is 99% different then RS4 except some screws.
(2) Sell your OS 18 CVR and get a real 12 engines (non pull start).
(3) Buy a real race ready 12 engine will slide carb, rear exhaust and SG crank...(again, non pull start and get a starter box.)
(4) Give up that rotary carb conversion, rotary carb does not performs as good as slide carb, it is a fact.
(5) Side exhaust does not performs as good as rear exhaust, it is another fact.
If you listen to the above 5 factors, you will have a pretty fast RC car to race or bash for under $600 (excluding radio and other stuffs...)