James,

My first microprocessor related job was using the 6502. The 8051 is light years ahead of the 6502. The 8051 has multiply and divide instructions (8 bit of course) which the 6502 does not have. Unless the previous design was using a math co-processor, I believe RCA 1802 or something along those lines, I think you will be ok using the 8051.

Good Luck,

Jacob