I would stay with a five or six speed freewheel at most myself-
but measure it all first-
theres also a nice little 3 cog freewheel usually on ebay for abot $12 shipped-
it might be easiest to try to get a wheel with a freewheel hub already built up- maybe even find some biogus used mountain bike on craigs for less than buying a whole wheel-
if you ran 700c or 27" on the back, it's usually A LOT easier to find a caliper brake for the bike- 700c gives you more tire options, but 27 tires are usually pretty cheap these days
Are you tal;king about a Thruster single fixie? Then probably the freewheel would screw on but you may need to get a longer axle and some spacers and maybe even bend the frame just a bit- that's why the smaller the better- You may have to cut a longer axle back down
but measure carefully- I wouldn't try to get more than an 1/8 " out of it- so measure first!