off the top of my head, here are a couple suggestions:
use a smaller (mod. 3/4) NWSL gearbox mated to small Sagami motor, with HO wheelsets on the axle(s)
there are small, self contained 2 axle powered units called 'Black Beetles' that are made in several wheelbase lengths
find a older Bachmann GE 44 ton HO model, they came with two separate power drive units (On30 critter modelers use these)