Off the shelf kits exist. There are also people who specialize in installing them. I think these kits don't take off because the people who are car nuts do some research about what type of performance can be expected from a conversion, and they conclude it would make the car worse overall. Worse performance, worse resale value.

Classic cars are only possible to collect at all because of a lower level of proprietary crap than new cars. If you convert a perfectly working classic car, you're throwing away whatever engineering goodness is inside it to install some inferior bespoke and likely proprietary system. You will probably have to cause permanent damage to the original frame and body of the car. What you end up with is a weird combination that could malfunction in some circumstances, and has inferior range and resale value.

I was thinking more along the lines of a simple/reversible modification like existing propane conversions. I know there are people doing body off Tesla conversions and the like.

I envision batteries + motor/bio fuel cell in the trunk or something. I believe Mazda is looking into electric motors inside of wheels/hubs and Honda was exploring hydrogen in push rod engines.

I come from a hot rod family, so I understand your point on classics. Perfect example is a T-bucket vs a Model T.

I see what you mean. Propane conversions are way less intrusive than EV conversions. Propane is not very heavy or bulky, and you can use the same engine in many cases. I'm no expert on it but that's what I've gathered.

I think hydrogen and other chemical power sources have the best potential to replace fossil fuels. I think Toyota is betting big on hydrogen, and they know a thing or two about EVs. Researchers are looking at synthetic hydrocarbons now too. Eventually, that may become the best option.