Singlespeed Conversion, Help Needed Please

Guys,

    could you help me please? I have an old 70s Bianchi racer that I would like to convert to singlespeed. I would like to retain as much of the original bike as poss. Can I get a conversion kit that would fit onto the freehub thats on the bike now? At the moment there is a 5 speed one on there but I would like to keep the old 27x11/4 wheel if poss. Also what number tooth rear ring would you recommend? Thanks in advance :-)