Having bought my khs there, i wouldn't do it again. (do they even still sell khs track bikes? other than the bunch of unsold ones the had a year ago 03 and 04 models)

At the time they seemed like they sold them, but didn't really want to deal with them. Also my cog/lock ring never seemed to be on right and i eventually stripped the fixes side of the hub.

as previously mentioned
for converted road bikes, yojimbos
for street fixes i would suggest Upgrade Cycle Works (suppose to be good custom wheel builders also), Cycle Smithy

also less central is
Boulevard Bikes, 2535 North Kedzie Boulevard in logan square. www.boulevardbikeshop.com
they seem to know how to convert road bikes to ss/fixed and have a basment of used bikes they fix up in the off season to resell. so if your looking for a converted road bike is also another option. They are right off the circle by the train stop.

there was at least one ally cat race that started there, they also support critical mass.