Developing a switch-mode power supply is fairly expensive, especially with all the regulatory and safety requirements these days, as well as the specialized (fire-resistant, isolation creep) components required. I don't think the volumes MFJ make are big enough to warrant development of an on-house power supply, not anno 2023.
Chances are that MFJ did not develop the power supply modules themselves but that they are OEM modules from another company and that they don't have schematics (most likely) or cannot supply because of NDA (also possible).
You should try to research it from that angle. What are the markings on the module?
And keep in mind that most SMPS modules are fairly similar. Typically, with some experience, a continuity tester and an ESR meter, you might not need schematics.