FSS Kits require the larger sear plunger/spring equipped SHB's... it's listed as a requirement on their website.
Couple of options, you can source a newer SHB and parts, or you can send yours into Apex and they will drill it out for your. Or you can drill it yourself, there are a few threads on it. They are inexpensive enough I'd suggest just getting a new one. Speed Shooters Specialties or G&R Tactical usually have them, or Brownells and Midway as well but they are out of stock often.
You can find them kitted or stripped - be aware that if you buy a stripped one you'll need the sear plunger and spring too
. Also a good time to add or remove a magazine safety if desired at the same time.