I don't know your engine is pull start or not, if it is, forget it, selling the RS4 3 with a almost new engines should give you a better price then just a roller (I would say $200 max)...
You better yet sell the car on EBAY over here or any RC related website, my experience told me ppl are usually asking MORE for LESS here then ebay, so you might ended up more then $200 on ebay....but I am sure not here.
If your engine is NOT pull start, go to tower and see you can get a slide carb (make sure the slide carb that works with your OS)...I am not sure how much (should be less then $50) so you need to think about is that worth it....that way, you can keep your engine since R40 does have a adaptor that will work with standard shaft.
Overall, R40 is a better car over RS4 3 in any angles....that should give you a much better start then RS4 3.
SPEED PASSION - King Golden, Speed Passion USA, Speed Power, Hirosaka Pro Products, East Coast Bodies, Schumacher Racing, Hurricane Motorsports, Serpent America, Sweep Racing USA. http://www.spautomobile.com/en/index.asp (SPEED PASSION AUTOMOTIVE)
NEW Speed Power/Speed Passion direct sale site - http://shop.speedpowerrc.com/