nios II implementation problem

I forget what 8051 code looks like, however the porting of the C code will mainly require changes to how you access peripherals, and whether or not your code was very dependent on what was previously available. So my answer is yes, but depending on the code it could be a simple or complex job. If the 8051 was memory mapped I/O (can't remember myself) then the port will not be too bad, and if this is not the case then some re-structuring may be needed (again it really depends on the code to be ported).