A project of an ICE for MCS51 was published in Byte review in years
before 1988. The schematic used now obsolete components, like 2716 EPROM, but with modern circuits, you can build a decent ICE without
pain. But the right component for this objective is certainly the
Triscend E5 with his inner programmable logic. Get a circuit with JTAG interface, wich implement directly an ICE function.