/jp/ - Otaku Culture

Does someone here know how to edit ".MAG" files or create new ones?
I've found this "ultimate collection" which is a pack with the emulator properly set up and an hdi file which includes all the games and a custom launcher in the same hard drive image.
https://spaztron64.github.io/th98tuc_site/index.html

It has some ugly stuff though:
- The games are only in the unpatched version, which isn't the best choice for a pack made for newcomers (the creator says that's because the patches aren't 100% stable)
- The games come with the extra modes unlocked and the Completed badge on all the shot types (the creator says it's for accessibility reasons)
- The game pictures in the launcher feature MMD art which doesn't really fit the games

The first two are easy to fix with some hdi editing, but for the third one (which is fortunately the last serious) I'll need to edit the image files, which are in MAG format (example https://files.catbox.moe/72ipv2.MAG)) and from what I've seen you need to use an editor inside PC98 Windows to edit them.
I'm looking for some editor you don't need to run on the emulator, hopefully someone knows about it.