You always have options with a G/S. Great platform for building whatever you want.
Yes, you can add the early GS subframe and get the solo seat...works best with the extended swing arm for weight distribution. Strengthen the gussets before powder coating
Or use the stock G/S set up..get the solo seat/frame from Germany..look on eBay for specific R80G/S parts...they should show up.
If you have a good welder, he can move the brake caliper castings on the fork lower so you can fit a GS caliper or other aftermarket setup. Must be careful since the aluminum tube can twist if too much heat for too long.
You've got a great start so have fun trying things until you get what you like!
BMW Club of Northern California, Ambassador, BMW MOA, Vice President Vintage BMW Club