Based on my understanding of old Sturmey Archer 3 speeds, a 7 speed internal hub could be problematic with a motorized bicycle because of the torque applied under load.

Even if you ensure that you get a hub/wheel that will handle the torque and work with your bike, you may want to consider whether you really need it or not, since the expense is going to be a lot greater than the $30 or so that you would spend to replace the derailleur and chain.