Just finished with this. To be honest I don't know if I'd want to try and get to the motor screws without removing the assembly? I have fat hands! Also, the motors I got given (ordered 2 in the end) I think had 4 mounting screws, whereas my OEM one only had three (unless... it wasn't oem).
Fiddly job and I hate messing with plastic trim for fear of breaking it. But I got there in the end and it's nice to have the window not taped shut.
One thing I would say is the aftermarket one I got is pretty wheezy, but it does work.