After looking at the 03468-90008 service manual a bit, I noticed there's no new signature analysis numbers listed for the 28-pin ROM 03468-65501, even though that's the default ROM listed in the parts list.
The signature numbers listed, and the pinout, are for the 24-pin ROM, 1818-3123.
Unless someone forgot to update the signature numbers, it could mean 1818-3123 and 03468-65501 have the same binary image. I also can't see any differences in the schematics between the two motherboards (24-pin vs 28-pin).
Not conclusive, but just sayin' it looks promising.
The older 24-pin ROM, 1818-1753, has a completely different set of signature numbers. So if Sailor has that ROM, it's not going to match images from either 1818-3123 or 03468-65501.
I am going to give it a try, don't know if I have enough time to get so far but will try a building a small converter pcb and a 27C64 in the actual device.
Now the question is does it matter if I connect the original /CE of the MK36000 to the 27C64 /CE or /OE they both have the same function as far as I can tell in a short glance on the datasheet?
I think *CE of 27C64 should be grounded, and *OE to *E of MK36000 signal.
The schematic for the 28-pin ROM motherboard has /CE and /OE connected together which then goes to /PSEN (would be the MK36000 /CE, pin 20).
EDIT: Clarification: No differences in the motherboards *besides* the 24-pin vs. 28-pin.