I'm sorry but, the PainKit has really been discontinued, because the way to create custom logo in P3D4 ( thanks to Render to Texture ), is so much easier, more efficient and flexible, that we had to get rid of the FSX PaintKit, becasue also due to the Live Update system, it would be very complex to keep the *USER* made logos updated together with the stock GSX objects.
The thing is, even if you manage to install your existing repaints, their SIM.CFG parameter will likely not work anymore with the current GSX, and the whole folder structure has changed, using a completely different way to indicate operators, even in FSX, so it would be really difficult and it will cause lots of issues if you tried to install them into the latest GSX.
Also, keep in mind we stopped doing active developement in FSX, and we announced that more than 2 years ago so, we keep FSX on strict "life support", even more so considering is going to have a whole new replacement from Microsoft in the next year.
So, as of today, if you really want to create new GSX repaints, you should use P3D 4.4 minimum.
