I'am thinking of a cheap dc geared motor off ebay and a pwm controller for the RP on the newt. You can get these motors for bugger all and in a ton of different ratio configurations, though for the price backlash maybe an issue lol.
http://www.ebay.com.au/itm/37mm-12V-...item415cb825f9 if you cant get a good focus with that and a pwm then your pretty much in trouble, only wind out times a issue though for a crayford you could just loosen side and lock down then adjust.