My bicycle is pretty much a fixed gear bike right now. Everything is new, chain, rear cog, wheel etc. The only thing original on this bike is the frame itself (yeah) and the cranks. The cranks are a 3/32 while the chain and rear cog is already a 1/8.
I've already dumped a good amount of money so far, and I think it's worth it since my friends have been helping me out with getting deals and selling/giving me their parts. Plus I get to enjoy building up my own bike.
I did the same thing with a '71 super sport. I used a Sugino RD2 165 crank and it worked just fine. Of course, you will need a bottom bracket of the correct length to get your chainline right.
Tom