well, it depends what you change, if you put a ION frame on it you can use lots of different triggers, but if not then the stadard sort of ION ups, bolt, board, grips, feedneck, eyes, detents, QEV, body kit. Considering the point of the EPI is to get better flow the firebolt is not the best bolt IMO, the bolt has to travel a long way in bolt terms, and with the amount of ports on the firebolt at those pressure the bolt is likely to break, and they do, lots of firebolts have broken in the EPI, maybe thats why they got rid of it
, but seriously, i would say get something like the tect T L7 bolt, they have less structual weak points and so will have higher tollerances when using high pressures. Smartparts put a new aluminium bolt in the EOS, so wait and see if they stick that on their site, i think that might be the best upgrade bolt for the Epiphany.
Edit : Deadlywind bolt doesnt work in Epiphany