I have a bunch of info on my website that should give you an idea of what's available and what your resulting speed/RPM should be.
Swapping gears within an existing FD housing requires a lot of careful work. If you can just buy the FD you want at an acceptable price, that's the way to go. Otherwise you have to totally disassemble two of them and reassemble (and reshim) at least one. That's a lot of work.
If you just want the GS fender mount on a different FD, you can change the cover pretty easily. I have never seen cover dimensions change by more than 0.01mm (they are all either 13.50 or 13.51mm deep), so there's no shimming issue. But the R11RS and K12RS use a housing without the standard ABS sensor hole, so you can't just buy any FD. I should probably add a section on housings and covers to my web page.