Quanky and Scrump
Quanky (left) and Scrump (right) are 1991 Yamaha TW-200's that have seen better days. Quanky's missing a few parts but has good compression in the engine. Scrump runs well but is still slated for quite a few repairs in the upcoming weeks.
Support, Chain
The part pictured broken on the left is referred to only as "Support, Chain," both in Yamaha's documentation and in online stores. It is a plastic piece brittled by age and broken identically on both motorcycles. It felt ridiculous to pay twenty or more dollars for a small plastic piece, so I set to work.
Pictured left is what I had to work with: the portion of "Support, Chain" that remained on Scrump (Quanky's was even smaller). In OnShape, my CAD program of choice, I imported an image of the complete part from the internet into a sketch and traced an outline using a series of straight lines and curved splines. The picture was not taken at exactly a right angle, so my tracing was not perfect, meaning that I had to clean it up manually. From there, I was able to scale my sketch to match the measurements I took of the part that I had. I then modeled in the holes and tabs. I did not include the recesses for the brass inserts or the cutaway (presumably originally for weight), as they would have made the print require support.
Miraculously, the dimensions of the part and tolerances of the holes were perfect on my first try. The next steps are to reprint the part with higher infill and thicker shells and give my replacement a long term test. The STL is available here.