Using legacy 2D meshes from BMP sources and setting alpha globaly

I have taken on a project from a 3D project of 22 years ago which was built with 3D Studio Max. The developers used 2D meshes as “sprites” and the source material 24-bit BMP files. As also done back then the artists used magenta for transparent color. I must be missing something but I would think there would be a way to designate magenta (255,0,255) as a global project alpha color but I’m not seeing such a setting. Other options are using code or even converting .bmp in the FBX of the asset to .png with an alpha channel and use the FBX SDK. Note there are a lot of these messes. Universal 3D is the setting of the project so a custom shader might work but I would like to know if there is a simpler solution.

Thanks for any suggestions.