I am sure, if it is possible to make them work for FSX, someone here knows how... very many talented people on SOH. Not sure if your gauge issue is with the model or if it could be fixed with a panel program like FS Panel Studio. I don't have that program yet, and have yet to mess with panels, but with a full size fsx cockpit build ongoing, it is coming.
My questions are this; #1 - Do you need the original files in order to make an older FS2004 model into FSX native format, and #2 - if you could de-hack the plane at the modeling level, do you need Tim Conrad's permission to do so? Which would be hard since Piglet (Tim Conrad) has gone quiet for many years, and I don't know if anyone here is in contact with him. So I don't know if that can be done. #3 - I think I tried the Helio Courier once and had a "squashed view" altered view format, which is from something they did in FS2004 that differs the perspective with the FSX aircraft, so even if someone can fix the gauge issues, can it be done for that aircraft in FSX? I know I got the same issue when I tried putting David Copley's XP-38 on fsx some years ago. But someone knows how, as one of his P-38 models were uploaded to Simviation without that issue, however that model had, I think two altimeters where one of them replaced another instrument, and it irritated the hell out of me. Once I buy FS Panel Studio, I may try my hand at fixing some old forgotten planes for Flight Simulator, and making some panels that people would really appreciate, that is to whatever limit that program can do. Should be interesting.
If Piglet was active on here, I would have messaged him long ago just to compliment him on some of his amazing aircraft. As a humble real world pilot cessna pilot, I have to say many of his planes have a "real" feel to them, acting like they really are up in the air. Many of them have unique handling qualities, and a couple of his WWII Japanese aircraft have a real "live wire" attitude and have no margin for error, which I found very enjoyably challenging. Where ever you are Tim, many of us love your work, and are still enjoying it, so thanks!