The March/April 2017 Gazette has an article on this very topic. Craig Symington is the author.
He tried a 25x12 motor but it was not strong enough. He then went with a Faulhaber 1319 but noted the high cost.
For gearing he used a NWSL 140-6 (281) but noted it was a little big and he had to remove some frame material.
This is a brass bashing article and he does a nice job of detailing the interior and exterior, including adding a backhead to hide the new smaller motor.
Lots of part numbers given. A real nice step by step article.