Cheapest is to make your own mount and use the 2 speed pinion adapter that is sold here on rc-monster.
http://www.rc-monster.com/proddetail...rcm2spdadapter
You can also look at the square motor mounts for use on nitro conversions at impakt rc if you don't want to make your own.
http://www.impaktrc.com/product_info...oducts_id=4256
Or the more expensive but very nice route would be products made by the dude.
http://www.tdracingrc.com/TD%20Product%20Line.htm