In the original IET documents, it suggested that bringing Mk3s up to regs only required an accessible toilet and compliant interior door handles. I wonder of fitting internal handles would suffice for the Mk2 sets.I think you can get away without power doors if you have someone supervising each one.
There's the visual announcements to consider too. Perhaps a set of pre-printed notices with someone to hold up the right ones at the right times? Or even a whiteboard?
A PIS could probably be fitted, but the issue with most of these things is time and cist