Guys I put an 19 tooth rear cog on mine, it really helps, I'll double check but I had a 18,19,20 ready to install depending on how it rode. It all depends on the brand of the hub, but most common wheels will be bendix style, its easy to find those to adjust the gear ratio. The front is a 46T, so you just need to change the rear.
Dmarks, your bike looks great. In the end it should weigh in close to 100lbs.
That pics looks like you have some wicked chopper bars/grips going on, if you look at it funny.