RE: Engines for 1/2, 1/3, 1/4 scale rc cars.
Half scale is pretty big, check out the Project Mammuth thread, that might give you ideas. As for 1/4 scale, you can use the same engines that are used in 1/5 scale. The higher end engines have more than enough power, so as long as your gearing is reasonable it will work fine. I don't know how much money you want to spend but there are a number of big block conversions for zenoah or CY engines out there, as well as some 60cc models out there. But unless your car is ridiculously heavy, a higher end modified CY or zen will do fine. There are also inline engines available.