I've got the Bulletproof BMX cranks on my conversion. They are available in many lengths; mine are 165. Cheapo Rocket chainring rounds out the package. A few minutes with a sharp razorblade and the cranks no longer advertise Bulletproof in huge block letters. They're not as pretty or as light as the track cranks on my other bike, but they get the job done with small cash outlay. With the ring on the inside and a square taper 107 BB, the chainline is about 41, which is right where my cog sits.