8-bit tools (and their modern counterparts) have far less gotchas than 16-bit ones. For starters, C compilers used to be really stupid and allowed unthinkable things to write (and to work!). Throwing a source code from the 80s at the latest gcc and trying to fix it may be harder than basically just reading the code and rewriting it from scratch.
Typical traps: 16-bit ints, different function bindings, different struct members, assuming order of instructions, ignoring the need of volatile and so on and so on. Way easier just to acquire the original compiler and one by one to update the code to at least ANSI C.