It all depends on what you want to do I suppose. I've come to realize that you can't polish a turd, it'll still be a turd when you're done. So if they are decent frames and you really honestly like them, and want to resurrect them as a fixie, then go for it. From what I undersand, the 1 piece cranks that have the huge BB shell are fine and can be used without problem, if that's all you've got. If you want to use a modern setup, then there are converters that'll go inside the big BB shell and give you a new shell to put a standard english bottom bracket in. But as long as the chainline is OK, use whatever bottom bracket/crankset you want. There's no harm in it.
Disclaimer: Have never worked on a 1 piece crankset before, although I have a few in my garage I have never worked on them. I have been doing LOTS of reading on bottom brackets and cranksets lately, and I do believe my information is accurate.