do the front cranks have one chainring or multiple ones? if it's one i would just leave it as is. you CAN get adapters to convert to a 3piece set-up, but then you are looking at the cost of the adapter, bottom bracket, and cranks.
if you are building a fixed gear then you might as well just get 700c wheels already built instead of trying to lace up the old 27in rims to a new hub. if you know how to build wheels then knock yourself out, but if you've never done it before it's a bit of an ordeal, especially calculating spoke length.
just make sure you get long reach calipers to reach the new 700c wheels, which are slightly smaller than 27", if you choose to go that direction.