I guess I didn't answer the question. I would want rubber.
Also - if the parts remain available, and are used in other bikes/equipment, then I wouldn't recommend the cost of getting some custom made.
The starter o-ring, for example, is not only used on other bikes, but also on Honda pumps. I'd expect that that o-ring will be available for some time to come.
If your price is under the $4 that Honda charges, then it remains a good deal, and would be an excellent addition to the kit. If the o-ring costs substantially more, due to the limited run, then I would vote to keep the o-ring out of the kit.
If you have other parts that warrant a similar investigation - let me know, and I'll do what I can.