A guy I work for has one, and it's on an old stihl demolition saw. I cant picture how changeover would work - you need an oiler on a bar, and none on an alpine magnum. The stihl demolition saws are awful to start, clunky and not very high revvers but pretty torquey. In all honesty though, nowhere near torquey enough so I would rather have the extra revs. The way most guys use alpines is to 'chip' rather than 'sweep'. Because you cant get a good sweep going, most guys kind of chip it down, and it bogs, lift it and it picks up, chip-chip-chip...
I've seen one run on a 3120 and it was way better. Having said that, those old demo saws go for very little money.....
Alpine magnums are just too expensive in aus to be worth me buying one. They're up somewhere around the $5k mark which is quite a lot for a mild steel wheel, a short section of extruded aluminum, a couple of pulleys, a v belt, couple bearings and some bolts. I've often thought of making one but never got around to it. The biggest thing holding me back is that I hate stump grinding so much.
Shaun