If you want to stick brushed then there are plenty of lathes going cheap, but I would recommend going brushless as the comm on a brushed motor burns quicker with lipo (they hold their voltage better than nimh during runtime)
Also you need to be careful not to over-discharge the lipos as the speedo doesn't have a low voltage cutoff (if the lipos go below 6V then they are scrap) so you need to fit this:
http://www.ebay.co.uk/itm/Novak-Smar...item4850a52ab7
The BEC on the GM will be fine, it's the newer speedos that have issues as they use a slightly different way of regulating the voltage that doesn't always cope well with big surges that some servos draw.