This is a really far-out idea, but if you can get the AVR assembly code (maybe from a disassembler)... You could try to find a suitable cross-assembler which will automatically convert most of the AVR assembly to 8051 assembly. Your job then will be to debug the resulting non-functional '51 code, which might be easier...!! :)) At least you will eliminate the step of having to make it work in AVR.

I could find a product on Google called "crossit" that cross-assembled 8051 code to AVR, but I don't know if it works in reverse.

Just a thought!
kundi