Has anyone tried these??

[doodlebug]

The duty cycle of the motor will be the big issue. Not all motor's are rated
"continous duty" The first one pictured, a heat sink could be attached to the motor, to help cool it. Reg is right, how much space will they take up?
But give it a try and let us know how it work's out.
It's great that you are looking for other way's, to build model's with!

Cheer's, Neil.