Realistically, unless your camera outputs a very clean high ISO image you're always going to be struggling. A 50/1.8 might just let enough light in to get useful shutter speeds but then you'll have no depth of field so all but a thin band of the shots will be out of focus anyway.
I tried a few snaps at the new Bury venue on Sunday, ISO2200, f3.5, shutter speeds 1/60 to 1/90 - a tad overexposed so could possibly have gone to 1/125 in hindsight - not an easy task on something as nimble as R/C cars and they don't really stand up to viewing at anything other than web sizes.
It's a damned difficult thing to do properly, pick the right spot for a panning shot or a hairpin so the cars are relatively static and good luck!