I was looking for some parts for my thunder tiger st-1 truggy and found these cups that might work
http://www.amainhobbies.com/product_...oducts_id/5328. I'd assume they would be the right size bore for the mugen, but not sure.
The brake discs should fit on them ok, because it sounds like the disks can be swapped between the thunder tiger and the mugen
http://www.amainhobbies.com/product_...ducts_id/11752