Scale Tank rebuild, what motors to use?

A few weeks ago I was struck down with the 'See it - Want It' virus, and ended up coming home with a 6th scale Challenger 11. Main battle tank for the British Army (think Abrams but with a tea urn).
At 1/6th scale this thing is big and heavy, about 80kgs with batteries. It runs on a pair of 12v motors but it can barley hit 3-4mph.
What I'm looking for is more like 8-10mph or more, but with the weight of this thing I can see the price going up with every mph over that.
If anyone can give me some pointers on a better system, brushed or brushless, I know its not the normal line of questions but i'm sure you guys can help.

cheers steve