Drill batteries by themselves weigh around or in excess of 1kg, seeing as the weight limit for beetles is 1.5kg there wouldn't be much left for everything else.

If you are really stuck for batteries i've got a couple of 1500mah 6v RX batteries that are free to a good home, in series they'll happily run a couple of those 1000rpm beetle motors, the cells will be better quality and they are much smaller and lighter.