Basically I am running 4 of these in my Bruder loader and compactor.
https://www.pololu.com/product/1586 I buy them from eBay though because it is much cheaper. If you want more power you could go with the high output version with a slightly lower ratio.
https://www.pololu.com/product/1577 The last ones are powerful enough to destroy their own gearboxes if locked up on 7.2v. I like the less powerful ones for this because they will stall without breakage and that also helps with steering.