After successfully converting an old road frame into a cyclocross rig, recently we have another similar assignment from Fabian L. Rompies to convert his steel track frame into a singlespeed cyclocross machine. Here's what happened along the way.
The frame was originally a generic steel singlespeed/fixed gear frame that happened to have enough clearance between the seatstays and chainstays to fit 35mm tires (plus enough mud clog). This is something you should consider before converting your frame, because all you have to do left is adding cantilever brake posts. Oh, and some cable stops if you prefer, just like what we did to Fabian's frame.
Fabian also insisted to use 700c road wheelset so he can have wide range of gearing choice, so we respaced the rear dropouts to 130mm. And because he wanted the frame to have that "works" raw steel look, we take our time with wire wheels and some fine grit sandpaper before coating the frame with a couple layers of clearcoat. His final gearing ratio is 42/18T, enough to tackle some of our local singletracks.
Fabian makes our job easier by using aluminium cyclocross fork taken from a Cannondale. If you can't find cyclocross fork you can also convert one from a road fork, as long as they have enough tire (and mud) clearance.