There was no need to double-post.

The DS5000T is a module containing the secure DS5000, a SRAM, the RTC and a battery; all in the "classical" '51 footprint.

From a quick glance, you could use a '51 derivative with enough internal code memory and internal data XRAM (I did not investigate how much of the internal SRAM is used but let's assume it's up to some 1kB or so, so it would fit into some of the '51Rx2), plus rewrite all accesses to the internal SRAM so that they map into the XRAM of the new derivative.

Or, alternatively, you could use a general-purpose (low-end) '51 (such as the AT89C51) in rom-less configuration (/EA tied to GND), attach external code and data memory in standard "Harvard" configuration; and remap the outputs from P2 into some memory-mapped latch (such as 74HC574), plus of course rewrite accesses to this port (which is not quite trivial given that nasty code does not use a symbolic name to access P2...).

In both cases, you would need to replace the accesses to the builtin RTC with a suitable RTC of your choice...

All in all, it just doesn't sound as a trivial task, and certainly you will need to rewrite some of the code, so maybe it wold be easier to start from scratch...

JW