In addition to the PIC12_6_8F offerings, there are also dsPIC30_3 and PIC24 PICs; albeit they are not covered by PBP. I barely glanced at the 8051 data sheet, so I know virtually nothing about them. I do know the Atmel AT89S52 is a direct replacement. Microchip recently acquired Atmel, so you can work with it in either the Atmel Studio 7.0 or MPLABX IDE (but not PBP).