Microprocessors are COTS items. The individual COTS components are built by the supplier into a bespoke LRU. There is no reason why a modern replacement equivalent LRU could not be made. This sort of thing there are loads of people out there with the knowledge to do that who maybe are clueless on older clockwork / analogue devices. Indeed, building up such a LRU can be much easier than attempting a completely 100% bespoke older electromechanical devices.
I routinely deal with this sort of thing within the rail industry - 10-20-30 years old electronics and processor devices through work. I know what can - and is done - out there- including (mainly in USA) warehouses full of unsold unused original OEM devices stored under controlled conditions that are available to buy. Covers all things like 8088 8086 286 386 486 etc, 68000, 6502, 9995, Sparc, and anything else you can think of, and proprietary ASICs, and on and on, and every form and factor of RAM, other memory and so on. There are specialist companies I have dealt with that can make you 1980s PCBs to 1980s spec (apart from now using lead free solder). It is all possible. A GB preservation group going to US and buying kit like this is no different to a.n.other GB group going to DE or RO for Maybach MD or Sulzer LDA engine parts or complete engines. If a group of GB people want to make a 60 work and have the mind set to do so, they will.
I can see there might be an issue with 'home made' devices when going for main lne certification, but go one step at a time, they need to work off main line first.