A knackered old Raleigh Twenty makes for a sturdy and easy to build fixie if you fancy the retro route; it takes a track hub with no trouble at all; the main thing to consider being upgrading the bottom bracket to ensure a reasonable gear ratio and chainline. Mine is fixed and is excellently fast, fun to ride and low on maintenence. The R20 frame is not the lightest but you never feel any flex due to it's bombproof construction.