Hmmm - interesting picture above - I assume that is one of the new style clutches. If so, it is different than the BMC V4 clutches that have sprags instead of balls in that gap. The BMC V4 has an inner race that you can't really see between the inner and outer disc pieces - I assume the MAC is the same. Anyhow, it wasn't really meant to be serviced.
I have ridden many many hundreds of miles with seized clutches. There is noticeable drag with a 26" wheel when not under power but not terrible. If you want to salvage that assembly, I would remove the gears, de-grease it (wash with detergent then optional final clean with spray brake cleaner), pop out the little clutch balls and bits, and clamp the discs together with a couple of C-clamps. Take it to a welding shop or muffler shop and have them throw three or four tacks in the gap - it'll probably cost $10.
The only problem I foresee is wear on the clutch key. A sloppy key fit really hammers the key from bi-directional torque that it was not originally designed to sustain. I had an original key that got chewed up pretty badly in that situation. It would be worth making up a new key with a snug fit if you are going this route.
The BMC clutch uses a 5v5x11mm key and I assume the MAC is the same since the clutches are interchangeable. You can go to a large power equipment place (snow blower, lawn mower, ATV) and buy a 2" length of 5x5mm keystock for a buck or so. Just file one end to fit (rounded), measure and cut to length (a bit long), then file the cut end for a snug fit.