The old Paintkit method is not supported anymore, since we have a far more flexible system in P3D4, which is way easier, doesn't require any editing, and it saves a lot of memory too and reduces the number of installed object in the sim, saving memory and startup time.
However, the new method is not supported in FSX, since it lacks support for DirectX 11 rendering, so you cannot create custom logos very easily.
I'm sorry but, since we announced to have stopped support for FSX in 2017, we must proceed forward to improve the product and use all the better features of the P3D SDK, which means using FSX will have some limitations.