according to sheldon brown, biopace cranks are fine and dandy for SS AND fixed gear use. I have an old trek steel roadie with biopace which i love. i dont know what it is about that bike, but it seems that the combination of parts plus the frame itself was taylor made for me. i have ridden many bikes, but this one just clicks.
anyway, i would try and keep the biopace cranks, i like them.