The chopper came about as a way to take a regular, factory production motorcycle and strip it down to create a lighter weight, higher performance machine that reflected the individuality of its owner.
Most now think of choppers as the gaudy customs churned out by famous builders and costing several times as much as a new factory bike, but it doesn't have to be that way. By purchasing a wrecked bike and mating its power train with a chassis kit, it is possible to build a complete chopper for less money than a new standard bike would cost.
Find and purchase a wrecked bike with a working power train. Harley-Davidson Sportsters are popular because they are common, not too expensive, and the engine and transmission are one unit, simplifying construction.
Remove the engine and transmission from the frame, and send it out for rebuilding if necessary. If your Sportster was an 883cc, you may want to have it rebuilt as a 1200cc for increased power. The 1200cc is the same engine as an 883cc, but with bigger cylinders and pistons, which are easily replaceable to upgrade to the larger size.
Choose and purchase a frame kit that you like and that accepts your engine. A few sources are listed in the Resources section. You can save money by purchasing a bare frame and a used fork, rather than buying a complete rolling chassis.
Remove the wheels, brakes and other needed hardware from the wrecked bike, and install them on the new frame. Any needed parts that were damaged on the wrecked bike must be replaced with new or used parts. If you did not purchase a rolling chassis, install the used fork.
Install the engine and transmission, gas tank and controls. Install the lights, seat and other accessories.
Test ride the bike to ensure everything is working properly. When everything is working to your satisfaction, take the bike to a paint shop to be painted.
Keep all receipts and the title for the wrecked bike for registration purposes.