Even just 1 gallon larger would be an improvement. That should get you a 165 mile range, versus 122 miles, as long as it doesn't ruin the looks.
I like the gas cap idea. Could design it to use more readily available caps, and caps that have varied styles available in the aftermarket, so everyone can find what they like. Could maybe see what styles are available in threaded caps, which would eliminate the need for threaded inserts. Not sure what's out there with the threaded type, besides the dirt bike market. Bayonet types are usually not interchangeable between makes, so you may have to pick just one, unless molds aren't a huge cost factor. I'd say Harley is going to be the widest selection in the aftermarket.
Painting plastic is no problem. Plastic can be wet sanded easily, and there's all sorts of plastic paints, even a chrome process, real looking chrome not just silver. Can probably prep a plastic tank quicker than metal, and plastic rarely has dents.
There's also hydrodip, available in more colors/patterns than a person can imagine. For the most part it's very inexpensive, quick, and easy. Cost increases when you have parts needing a larger dip tank, like atv bodies, which I'm about to try. I'm on the lookout for a 12' EZ-Up swimming pool.