I've seen these 4 stroke engine conversions for the tmaxx. They say that once you buy the kit you need an O.S. FS40 4 stroke engine. I've looked at hobby sites and I see an O.S. FS40 4 stroke engine for both airplanes and for 1/8th scale cars and trucks. What is the difference? Can I use either one? Because I have found the airplane ones on ebay for around $50.