Dunno what that RAR business is, can you post it in a standard file format?
If the 8051 source code is in the C/C++ programming language, it may be possible to port. If it's some other language, you will end up re-implementing the code from scratch in C/C++
Here it is as a zip (BTW, RAR is a standard format)
Personally, I would probably try to get a standard arduino library working instead, using the demo library they gave you to help you work out what you need to change in the standard library.
Ah, so it is just plain C. That makes things easier. Just have to change the hardware-specific parts (like SPI) to Arduino equivalents, a few data types (like 'short') to Arduino equivalents, plus a few other odds and ends.