After measuring the frame some more and shopping around for materials I decided to start the CAD work all over again. The first two designs were too cramped or involved fairly expensive materials.
This latest design will be easily made with less than $60 worth of steel (L-channel and 1" square tube) and some sheets of polycarbonate (which I buy as scrap). All while allowing for a wider range of sprocket sizes.
And it will only require me to cut out and replace the small downtube that held the engine and skid plate, rather than having to alter the main fork of the frame.