As anyone who's seen “American Pickers” on the History Channel knows, vintage motorcycle parts can be harder to find than the bikes themselves. From forks and fenders to saddlebags and speedometers, finding that elusive vintage motorcycle part for a treasured Harley-Davidson or Indian can be a collector's dream come true.

Harley parts are in a class by themselves. Sometimes the need is simple—a replacement petcock valve, for example, between the bike’s gas tank and carburetor. Other times, the part needs to be decorative as well as functional—maybe you need the gas tank itself, or you’ve always wanted to attach a sidecar to your ride.

While do-it-yourselfers routinely seek frames, handlebars, and headlights for Harleys made in the late teens, the ’20s, and the early ’30s, parts for Knuckleheads, manufactured from 1936 to 1947, make up a sizable portion of the vintage Harley parts market. Sometimes the prize is nothing more than an authentic emblem, but serious mechanics are often looking for original cylinders...