I haven't built one but have seen a few. A microwave turntable motor from a scrapped microwave (I have collected several of these from junked microwaves, often you don't even need to open the case to get them), or a rottisserie motor can be used to power the things. Check the voltage on the microwave motors as I fried one when I failed to look and it happened to be 48 v, most are 120 v though. many swiss, grana and pecorino makers seem to stir with the curd cutter since they are looking for rice size curds. Or a paddle can be rigged, just remember it should probably have a lifting action to turn the curds over as it stirs. and initial stirring may still need to be done by hand to be gentle enough to not shatter the new wet curds. Show pics if you come up with something please. I haven't given this serious consideration since most all of the cheeses I am interested in are not stirred much or long.
Good luck,
John