(I understanding P89Cc668 doesn't 100% pin compatible).

P89C51RD2 and P89C668 are totally pin compatible; HOWEVER, the SFR addresses are, in many cases, different.

I went through this process ~18 months ago and, after sorting out some SFR errors in the .h file the "compatibility" was 100%.

A little trick with the 668 (and SILabs derivatives) if you use the esacademy CodeArchitect for LPC, you can bgenerate all the code for the hardware IIC, all you have to change is the clock registers and the SFR addresses.

Erik