It's less about getting the 1995 hardware working, and more about rebuilding the actual time server to serve the original purpose, for the next 5-10 years at least.
I'm planning on using this chassis to build up an NTP Stratum 1 server for the global NTP pool, hopefully later this year. I thought it would be a lot more fun than letting the box go to e-waste.
And leaving the guts completely untouched means I can do a full restoration someday, if I want.
The two main things I'd need to do:
1. Modify the RF chain so I can use normal GPS antennas that don't down-convert the frequency.
2. Modify the DC bias voltage so it doesn't fry GPS antennas.
After that, I could work on patching firmware, but I'd probably just use the original guts as a frequency reference and leave time to other hardware.
I could even run the Pi and the old hardware simultaneously. The old hardware providing IRIG-B, 10 MHz, and 1 PPS references that are a little more accurate (it has a TCXO, haven't measured its timing accuracy yet). And the Pi could provide PTP and NTP, using a more modern GPS chip which even supports dual band and survey-in for more accuracy.