Jon,
Depending on which scale you're working in a conversion to AH-1J seems to be a fairly straightforward job. I was a 68M20 from '80 to '87, and worked on the AH-1S Prod, Mod, ECAS, and MZ (F) model Cobras. I also got to do so work on the AH-64A, but would've had to gone back to school to get qualified on them.
Anyways, the AH-1J was basically the AH-1S ECAS version built for Japan. The main differences between the J and S MZ (F) were the exhaust suppresion system with the disco light (IR jammer), the ADS (the boom and probe coming off the right canopy), the LASER tracking, and some mainly internal systems
Using the Revell 1/32 kit as a starting point, you will basically have to scratchbuild the flat panel canopy, cockpit, the IMRB/K-747 rotor blades, various antenna (transmitter/receiver), as well as the XM-197E1 UTS with the M-197 20mm cannon.
You would also have to correct the angle of the wings, as the AH-1G Revell kit's is much to steep. Scratchbuilding of the M-65 launchers (TOW) is another step.
I believe these are MOST of the changes between the G to J, but I'm sure there are some others I've forgotten.
HTH<
Jeff
HTH,
Jeff