There will be no work to do to the receiver by you. Early GS receivers had some post-production work done by Omega, but you should leave the semi receiver alone. UMP parts need to be modified to fit. UMP bolts need to be milled, UMP Lowers need to be converted to semi and have hooks opened up for semi receiver ( true for USC or GS )
TBT is also building on the Omega GS receiver. Check out https://www.atlanticfirearms.com and search KSM or Tommy Built
Also, best deal on a USC that I have seen lately is from supporting vendor hkspecialist
Or, you can lurk around here and GB, source a USC barrel, a bolt, a lower, and the other little bits to put together a GS receiver.
HKspecialist also has a pretty good deal on UMP lowers.