RE: Engines for 1/2, 1/3, 1/4 scale rc cars.
A big fat chainsaw would work with the right gearing as they have heaps of torque.
Or a motor from a demolition saw or something similar.
Anything 50cc or over.
Or you could look at a 2-stroke lawnmower motor.
They're usually 100cc or more.
It'll take some engineering though as you'll need to get a clutch on there.
Maybe something from a scooter with a cvt?
Gokart motor......?
Unless you wanna go 4-stroke and look at a Honda/Kawasaki motor or a Chinese knock-off one.
Depends how fast you want it to go and how much ya wanna spend....