... On most adjustible fork bikes there is a hole in the bottom of the axle to adjust compression dampening. You will need a new axle in some cases but not all....
Understood, but the compression/rebound adjustment on decently engineered modern cartridge forks (which I assume is the object of a swap) helps tune the suspension for rider preference & track (road) characteristics within the relatively narrow range of weight/weight bias of the bike for which it was engineered.
...as far as the internals, most sport bikes are front weight bias so they are almost always too stiff for monsters unless you happen to have the correct weight.
Bummer (I’ve not compared monster to other sport bikes in terms of weight/bias). So if the donor bike has significantly different weight/bias, not only the springs but the whole damping setup would be wrong for the recipient. If you’ve got to re-spring and re-valve (what I call "internals") to get it good (that’s $300+ for everything using the original cartridge – Race Tech valves/springs for example; $800+ if you replace the cartridge with Traxxion AK-20s or something similar), why swap as opposed to just upgrading the OEM fork internals?
Again, my assumption is that if you are doing the swap, the object is to get a modern suspension that is better than OEM for your situation – not to get a ‘different’ suspension that’s not optimized for your application.