Since you are most likely to be operating these speeders as a "hobbyist", and not as an employer, I don't think OSHA "Occupational Health and Safety Administration" applies at all.

Since we haven't built speeders in a while, I suspect that all are exempt, for most part, if not completely, from any current emissions rules, and/or inspections. Like older cars and trucks...they're probably pretty much "grandfathered" in, based on the rules that applied at the time of production. But, Mook might be able add more info for any CA specific variations on that, if I'm wrong.

Even liability insurance requirements, most often regulated by states, would likely vary state to state, and even property to property, on which they were operated. Although, I'd sure require that, at a minimum, on any property which allowed such operations. And so, I suspect most folks who do allow speeder operations, do require some kind of proof of liability insurance. Sammie or Howard will probably have a pretty good idea what they'd want to see in that regard.