Peter,
There are multiple levels of customization available:
1) Bind a new terminal gotxy (mainly aimed at using a different VDU connected via a serial link as the console) eg VT100, ANSI, ADM3 etc
2) Change the disk parameters for an existing build - number of blocks, SPT, no reserved tracks
3) Use the CP/M adaptable system. This boots CP/M you run a program to extract the CP/M bios which you can then use to create a P-system bootable disk (you only need CP/M as part of the adaptation process) This has the advantage of minimum effort though it also gives your worst performance as there is a shim layer adapting the SBIOS to the CP/M CBIOS. You can also create CP/M programs to perform block reads, writes and disk copies to help you along the way. There are 2 CP/M variants those for 8080 targets and those for Z80 targets.
4) Use the full adaptable system and write your own custom SBIOS, primary boot PBOOT, secondary boot SBOOT, and transfer utility for the target system. Also assumes that you have a working 8" IBM3740 (250 kb FM disk, 128 byte sectors, 26spt, 77 tracks). The Adaptable system comes in 2 different formats: 494 block (247 Kb Standard Volume) and 3 volumes (each 26spt x 25 tracks) packed onto a 250 Kb 8" IBM3740 disk. You will need to write your own utility to copy the blocks from the volumes onto a target disk eg 5.25" 160Kb MFM disk.
5) Write a new P-code interpreter for a new target CPU and hardware platform + SBIOS + PBOOT + SBOOT + utils.
When you have finished your adaptation don't forget to run the new system through the Validation test suite.
Hope this helps
Peter
Much of the above is easier if you have a working platform to develop the new code on. Most standard UCSD P-systems can process a Standard IBM3740 8" 494 block volume and packed 3 Volume versions. You also have access to working Assembler and compiler.