I've done a conversion on the bruder cement truck body:
I found the easiest way to drive the drum was to hide the motors in the water tank moulding:
Check out some of the robot parts suppliers for gear motors and other mechanical parts. These guys have an incredible range, they are in the UK but ship globally at good prices.
http://www.technobotsonline.com/inde...ategory_id=176
I have found that the Pololu micro metal geared motors worked well for me, small enough to hide and plenty of torque to drive the drum with a couple of kilos of load in it. I used 2x High Power (HP) in parallel:
http://www.technobotsonline.com/moto...ed-motors.html
but you could use something else if you are going for the external chain drive.
The same place also stocks mechanical parts like gears and chains, make sure you get a matching set so everything works OK...
http://www.technobotsonline.com/inde...ategory_id=193
as for speed controller, I used this one because it takes up almost no space:
http://www.robotpower.com/products/wasp_info.html