Re-encrypting them isn't terribly hard, the address and data swaps are static, and mame already has good handlers for dealing with those. The REAL problem is every single pegasus software rom (and the bioses too?) has a serial number embedded in it which is compared against the serial number set on the machine using diodes (and sealed under some epoxyish white putty stuff); if the numbers don't match, the software doesn't run. The bios can apparently be patched to remove/bypass this check, but a better way would probably be to auto-patch (IN RAM! not permanently) every rom to have the correct serial number in it, so the bios checks pass.
"When life gives you zombies... *CHA-CHIK!* ...you make zombie-ade!"