Zack,
Have you read Sheldon Brown's articles on fixed gear? There have been some more or less successful modifications of freewheels to fixed by welding the pawls or something. I'm not sure why anyone would want to go to the trouble. What you can do is take off the freewheel, screw on your track cog, then use a standard bottom bracket lockring to lock it down. A number of people use this setup successfully, but it has to be done carefully. A real fixed gear hub has standard clockwise threads for threading on the cog then outer counter-clockwise (left hand) threading for a track lockring. When slowing down a fixie by resisting the forward motion of the cranks you put tremendous "backward" or counter-clockwise stress on the cog. A left hand track lockring prevents the cog from unscrewing because this backward stress actually tends to tighten the lockring. When using on bb lockring on standard freewheel threads, there must be VERY good lockup to prevent the whole setup from unthreading. This does not mean screwing both cog and lockring down as tightly as possible, but screwing the cog on good and tight then the lockring then tightening the two against each other. Sorry if this is embarassingly elementary since this is how all lockrings and locknuts work, but I'm never sure if everyone quite understands this concept since one can often get away with simply tightening the first cup or nut down tight then tightening down the locknut/ring. I sometimes wonder if this is why some people seem to have trouble with headsets, hubs, or bb's regularly getting out of adjustment. On these parts the problem is just one of adjustment. Failure to achieve a good lockup on a fixed gear can result an a sudden unthreading of lockring and cog - not a good thing.

With regard to cranks, yes, you can just use one ring on a double crank, but you need to get a set of short stack crank bolts because regular crank bolts will be too long when you remove a ring.

Hope this helps,
Raymond