Prereqs
 

Is there a way to make a Magus arcana check for a certain archetype, so that the arcana is restricted to that archetype?

Just use a prereq script that checks the hero for that pick.

Could you tell me how to do that or point me at an example that does it, so I can extrapolate myself?

quick example would be cNinArcaneBackfir

fDecoyMisd is an even closer example

lol now can you tell me what pack those come from or at least where to find them in the editor? The first I assume is a Ninja trick which I couldn't find among the custom abilities so I am uncertain if I have the pack it came from and the second looks like a feat though I can't find it there. Am I looking in the wrong areas and if so, where should I be looking?

Ninjas and the Arcane Backfire Ninja Trick are in Ultimate Combat, and I'm guessing the second is Decoy's Misdirection from the Familiar Folio?

Arcane backfire is from Heroes of the Fringe, and I don't have the familiar folio

Go to the feat menu and type in "archetype" anything that requires a specific archetype should appear.

Thanks Minous that worked