After you have disassembled your Mag 21 you will notice that there is a long top out sleeve - cutting down this top out sleeve is what you need to do to increase the travel of this fork:
It's up to you to decide how much you want to take off the top-out sleeve (and accordingly, how much extra travel you will get). However, if you shorten the top out sleeve too much the fork stanchions at the top of their travel may clear the lower bushing. This would be a very bad thing.
I removed about 15mm which bumped the travel of this fork up to around 60mm. I say "about" because travel is dependent on home much pressure you put in the fork, as this compresses the top-out spring (which nowadays would be called the negative spring). More pressure (for a heavier rider) will compress the top-out spring more and lengthen the stroke of the fork.
Top out sleeve before:
Top out sleeve after:
