This project is based not on 8051 but it uses the AT90S8515, so running this hex file through common 8051 disassembler will not produce anyting usefull.
Correct me if I am wrong, but for me this will have to be done "by hand" having both microcontrollers, the AT90... and 8051, sets of instructions and addresses of bits and registers ..
I really don't think you will find any simpler way of translating this code to the 8051 sourc file ..
Good luck and regards ..
IanP