Rob,
Dunno if you’re still looking but we’ve 2 Fattys in the family & recently converted them to 2 x 10. I used Truvativ Holzfeller 2.2 RG Chainset & BB MY10 – 170mm – 22/32 Rings – White from Merlin Cycles. I used a standard front mech off another bike just at a slightly strange angle, but it all works just fine.
Re the Fatty in general, it’s great value for money & fantastic fun in all conditions, snow, sand, mud, rough rooty stuff or just popping down the pub. We got fed up with the Avid brakes, they needed bleeding a lot so changed them. The wifes bike is running very well but I’m a bit harder on mine & have done a set of rear wheel brgs & my BB is now horrid! but most of that has been caused by the sandy conditions in the Surrey Hills last winter, had them since last December.
Hope that helps a bit :-)