Today was a holiday, and I had a bit of free time, so I built up the drive wheel:
That's a 52 tooth chainring on a 20 inch (406mm) wheel.
I'm not sure why more people don't build their own wheels. You do need a truing stand, but other than being a little time consuming, it's not too difficult.
I also ordered the Breezer dropouts which I'll braze into the old Mongoose rear subframe.
This is the current rear dropout. "Opus the Poet" suggests that I just spread the dropouts a 1/2 inch rather than building a whole new rear subframe. Good idea. If that works I'll cut out the rear facing dropouts and braze in a Breezer style dropout.