"I have had bad experiences converting non-functional code to another processor, so I would like to make the thing work with the AVR before converting it."

I could supply you some non-functional 8051 code, if you want... ;-)

Seriously though, this is certainly the approach I would take (or get the original supplier to do it?)
I guess that you already know the standard tricks of isolating all the compiler-specifics into typedefs and #defines - I have often used this to make embedded code compile (and even run) on a PC for testing, etc