There are several chips in the Apple 1 that are not very available nowadays, so it seems wise to provide reproductions for them. The chips might include:
- Signetics 2504, 2513, 2519
- 6502 with ROR bug
- DS0025 clock buffer
The generic TTL parts will probably have good availability for quite some time, so there's no need to worry about those.
In low volume, one way to go is a small DIP-sized PCB with an FPGA and any appropriate power and signal-voltage translation. This would plug right into the target socket. As a start, I've done the HDL and PCB layout for a PROM board, available here:
Attached is a screenshot of the board and a schematic PDF. You can see the holes for the 16-pin DIP pins along the edges of the board; eventually it might be nice to replace these with a DIP lead frame (tin-coated 260 brass?) that would offer more realistic pins.
This is a work in progress and nothing has been tested yet. I've sent off the small boards to OSH Park and should get them in a week or so. There might be showstopper errors in it.
I've also included in the github repository the schematic and layout of the Apple 1 cassette interface. That is also untested (I don't have an Apple 1 yet). If someone wants to fab and assemble one of those and test it in an Apple 1 replica, that would be great. I would suggest the ENIG finish for a prototype board---it's not as durable as electrolytic hard gold, but should be fine for a quick look.