Hiya, welcome to the forum!

Most antweights use motors that are smaller than those - 12mm Sanyo-type gearmotors from Solarbotics and Pololu - or a few robots are still on modified R/C servos. The weight of those motors (30g) could be prohibitive though, and the current draw limits what components you can use as of all the currently used controllers only the Scorpion Mini or Sabertooth 2x5A would handle that current safely, and your batteries would have to be able to provide the current as well. (also some of them are rated at 12v and above)

If you could use them with some good tyres and get within the 4 cube/150g limit though you'd have something quite powerful on your hands

P.S: I'd advise you to check the antweight forums as well, as quite a lot of ant builders frequent those and not the FRA boards (the ant events being run outside of the FRA's jurisdiction): http://www.robotwars101.org/phpBB/index.php is the place to go ^^