The pump head doesn't wear and if you don't disturb the pump head, you don't need to bleed the system with the factory sd2. Replace the motor only in situ.just in case...thanks!
But if the parts are not beeing sold seperately, i don't see any meaning in replacing only the one part.
I haven't done this myself and therefore I can't say how easy or hard it is to replace just the motor in situ.
Regarding just buying the motor, you could just buy the motor alone, I've seen sellers in the US on ebay, a few years since I last looked.