When I worked at an auto restoration shop 15 years ago, all we used was 2 part acrylic enamel. It wasn't as tough as poly paint like Imron, but safer to use, and could be color sanded. Dupont Centari was a favorite brand I remember... It's still around today, but not cheap at $100-$150 a gallon. I'm sure a quart can be had, and should be enough for a bike... Maybe a local auto paint repair place has an interesting color of similar paint they can sell you cheaply. Sometimes they order too much for a job, and the extra just sits in their paint cabinet. Anyway, I vote for acrylic enamel, or if you have the proper spray & breathing equipment, a polyurethane like Imron.