Could be tapered too. Although you could always put a reducer in there and still use a 1-1/8. Just don't buy something bigger than the headtube (1.5 or tapered if it's 1-1/8, for example).

A fork with a longer axle to crown measurement will slacken your bike out. So it'll be more stable on downhills, but not as good of a climber and slower to turn. Which could be fine if you only ride ascents to get to descents...I put a longer fork on one of my bikes and now it descends like awesome-sauce. Like DX said, you can go with a little more travel, but don't try to put a 160mm fork on there...it'll stress the headtube and mess up the steering. Also keep in mind that pretty much anything you put on will be heavier than a SID, so the balance of the bike will change to be a little more front heavy.