It seems for the most part, this forum has been abandoned for the more popular system flash cards. I have the same problem but have found some success using PCETOY (Google it). It hasn't worked on all my US roms, but it has on a few. I wish I had a better solution, but I don't.
There was a thread on here that mentioned that you can use a hex editor to modify a string of values on the US ROM, but I found most of the games I had did not have this string.
This is't the fault of the Romcart tho. You can blame NEC for region locking the PC Engine. If you were on a TurboGrafx, it would play all games without a hitch, at least my old one did...