see if you can find a reducer at a hardware store...i'm sure there has to be something you can use in the plumbing section (though i'd search long and hard for something silicon rather than rubber - rubber expands under compression more than silicon - but you are n/a so you won't get have a huge amount of compressed air)

correct me if i'm wrong but i think mugen changed their name after a scandal by its management...i can't remember their new name but i don't think they market under "mugen" anymore