The level format is completely changed when loading a vanilla (original) SMBX level in SMBX 1.4.x, so versions like 1.3, 1.3.0.x, the PGE Editor, and SMBX 2.0 cannot load it anymore. Well, for me it won't load a 1.4.x level in anything but 1.4.x, so that's all I know.
This also means anything I share that's SMBX related will be for the 38A version. You are more than welcome to make masked versions of my graphic submissions for any versions of SMBX that use sprite masks. Please just let me know.
Come join the SMBX 38A Server!