Compilers should just add a --k&r mode to appease these people, but it should also reject ansi code with parse errors.
If speed is not a requirement for the --k&r mode, you could just take the tis-interpreter and note that if it runs without UB, it is still much faster than an actual computer was when k&r were active.
Would it even be possible to specify a variant of C that contains no UB (e.g. would define exactly what happens on unaligned access), but can compile practical existing C89 programs? I wonder if it could be written such that it could actually specify the behaviour consistently across the language intersection supported by both of e.g. GCC 2.95 and Chibicc[0].
Or maybe there are so many bugs in GCC 2.95 that it would simply be infeasible? How much time would it take to specify?