Seems like taking a major chance to me. Even if you could source the motor and all the other pieces you'd need individually (Shimano is kinda difficult to source spares from), you'll probably pay more than just buying the complete Alfine Di2 hub with shifters. I'd buy the whole mess, and swap internals so I wouldn't have to re-lace the wheel (if possible). Then I'd put the old hub internals in the new hub case and sell everything left over to recoup some of the cost of the new hub. If swapping internals isn't possible, I'd just make a new wheel with the new hub and sell the old wheel with its shifter, cable, etc..