There is a metal sleeve (top out sleeve) that sets the travel (well mag 21/10).
Cut this down accurately and you can set your travel to whatever you want. That is all the long travel kit really is. I suppose you could tinker with the ports to make better use of the long travel and spacer the springs, but just look in the sutherland mag service guide for you weight and travel settings for various ports and washers for tuning.
But rather than explain in full, read this and it should make sense. There's more there than I've ever tried. http://206.75.155.18/kmr/mag21.html
Add a pic
It's the long silver bit above the spring and below the bushing
MAG20 internals on the other hand are different
The two silver parts house the bushings, I guess it you trimmed these down you would increase travel, but loose a lot of bushing separation and probably make them quite fragile as the two narrow thicker sections where they touch wouldn't be there any more for support
I know - but at this time of the morning I sometimes can't resist the temptation, Devils advocate and all that...
I just Googled it at the time and copied, not altogether accurately as it turned out, what I found. I'm a long-term Rockshox fan myself, but not in the same league as Ben though...
Good question re the long travel too, I shall have a play around with that myself - Cheers Ben...