So I looked into this conversion a little further today. and I'm going to say it'll be difficult and probably ugly if at all possible.
While the PCBs are "nearly" identical and the protection chip is the same Raiden DX uses program ROMs twice the size of Raiden II.
Raiden DX uses 4x 27c4001 EPROMS while Raiden II uses 2x 27c4096 EPROMS.
the Raiden II PCB has solder holes available for 4x 32pin EPROMs, so my initial thought was to remove the 40 pin sockets for the 27c4096 chips and install the 32 pin sockets for the 27c4001 chips.. looking at the masking on the PCB for these chips though they're labeled "27c020" and indeed checking the pin locations the address line necessary for a 27c4001 EPROM is tied to +5V. In short this means that they wont support the proper sized ROMs for Raiden DX.
It looks like the ROM address lines are run to the Seibu custom at U0135 (SEI1000 SB01-001). I believe this is the protection IC, but it's unclear if the version of this chip installed on Raiden II will suppor the higher address lines, and even if it does it would mean running some wires from the program ROMs to this chip to "attach" those address lines. We likely would need someone with a Raiden DX PCB to determine which pin(s) these address lines attach to. Also there may be some other differences related to these address lines that aren't immediately apparent.
looking at MAME source there are some differences regarding the memory banks, it's possible a patch would have to be made to accommodate this.
for now though this conversion seems like it would require more adjustments to the PCB than I'm interested in performing.