I would assume FPGA as the origianl custom support chips : ANTIC, GTIA, SALLY and POKEY aren't mass available. One guy I saw online in 2017 was selling chip pulls from excess BallBlazer carts from the 7800 he had.
I thought so too, but it could also be software emulation...
OTOH, in the Commodore scene, there are lots of replica chips available these days so that you can build a complete C64 from scratch using the original Motherboard (and even on the Amiga some of the rarer chips are being re-created, like the Agnus)
Of course, that's not much different than using an FPGA to just emulate everything, but the one-chip-at-a-time approach has the advantage that you can swap in original chips that you might find, and only replace those that you're missing...
https://x.com/rm_800xl/status/1714336578619707408
https://forums.atariage.com/topic/268697-how-rare-are-atari-.../
OTOH, in the Commodore scene, there are lots of replica chips available these days so that you can build a complete C64 from scratch using the original Motherboard (and even on the Amiga some of the rarer chips are being re-created, like the Agnus)
Of course, that's not much different than using an FPGA to just emulate everything, but the one-chip-at-a-time approach has the advantage that you can swap in original chips that you might find, and only replace those that you're missing...