I have one of these in the garage. I think they're 6502-based with the OS on EPROM, or at least that's what I vaguely recall from last time I had mine opened-up about 20 years ago. I can dump the ROM if it'd help.
There was some bad blood between Wa8DED and NORDLINK/TheNET about copying, maybe best to contact WA8DED directly, or the NORDLINK guys.
https://w5yi-vec.org/W5YI_Reports/1988/1988-06-01_W5YI_repor...
Then again, maybe it would be better to jump on the LoRA bandwagon and make something similar work with meshtastic: https://github.com/TheCommsChannel/TC2-BBS-mesh