your PUs can be saved!
hi, i have a bit of experience with both epi n gibby firebirds, so maybe this might be useful:
1) a PU change is going to make a HUGE difference in how the epi sounds, the stock PUs are really poor. i've tried the duncan ones before, they make direct mini-hum replacements and there are 3 versions, go check the website i've tried the seymourized one, its really nice.
2) if you cant afford it or dont want to change PUs for some reason or another, there are 2 other mods i feel are essential with the epi firebird: get a good luthier (malcolm is who i'd recc) to change the nut to a bone nut, immediately your sustain and tone will improve.
3) also, you must get someone(for this beez is best) to change the caps (capacitors, beez has got stock of some really nice orange drop ones and they are not expensive) on your tone or volume knobs (not sure about this beez will know) to a lower value. sorry i cant give you the specifics cos i'm not a techie but many years ago a mad useless dishonest opinionated prik of an aussie luthier in s'pore called mark (marc?) stevens (stephens?)did it for me and it was the one decent mod he ever did, the cap change really cleared and brightened up the stock epi PUs, i'd go as far as to say the mod saved the stock PUs for a long while i was happy with them until i finally changed to duncans, which is what i still use with the gibby today.
4) i dont really recc the fabrication of custom parts for your PU rings and stuff, the size is really funny and you wont be able to do it easily. why pay for duncan single-sized hums when you can spend a bit more and get mini-hums the size you need exactly? plus they dont sound exactly the same: the single-sized hums sound good but mini-hums have a unique sound all to themselves, they're fat like real hums but not as smooth, brighter and with a bit more punch, so they're worth keeping. also, there is no way p90s will fit into the routed hole on your epi.
dont sell it tho. the firebird is a special guitar and you either love it or hate it, but if you love it like i do then its really something else, and i feel that with some money spent, the epi firebird can turn into a really rocking guitar that you can gig with no problem.
hope this was helpful.