How should I gear a 12" wheel on an adult bike?
Using an 8 or 9 tooth BMX cassette hub will get you into the normal useful range for singlespeed gearing pretty easily with a normal large road ring. 53-9 on a 12" gets you about 70 gear-inches for example, which is a common choice for fixed/singlespeed road bikes. Most present-day freestyle BMX rear hubs would work fine. (Most are 9t, some are 8 - 8s wear out fast and are considered going too far by many.)

Using a 56t tri ring and a normal 12t cassette cog gets you into around 56 gear inches, which would probably be fine for a chopper. 56t rings, while not exactly common, are easy to get. Origin8 makes a 56x130. If this gearing worked for you it would probably be a cheaper route.

Since you mention going on longer rides with it, the good answer may just be to run it with the Shimano Capreo group and a 56t ring, giving you the benefits of a derailer drivetrain. I'm pretty sure the Capreo derailer should still clear the ground on a 12", but you might want to corroborate that somehow before doing it. It may not on an 8".