The ADuC812 has hardware dedicated to I2C support, including the I2CDAT and I2CCON sfrs. While most is for hardware slave even their I2C apnote uses some of this hardware for software master. The 89C52 doesn't have these sfrs. If your code for the 812 depends on any hardware besides the port pins, it won't work on the 52.

Compare the sfrs between the two chips, checking your code to make sure you are only using the common sfrs.

You might check instruction timing also; the 89C52 is a vanilla 12 clocks per instruction, I'm not sure about the 812. Differences will affect results if your code is pushing timing limits of I2C.

Dennis