Peter.N. wrote:Where is the energy going to come from to produce the hydrogen?

Exactly! But it seems that any car running on fossil fuels is going to be a classic according to Q Willson¹.
I suppose it will mean more nuclear power stations (where's the uranium going to come from because I don't think we have any naturally to mine?), more renewables (and presumably fossil fuels to manufacture the raw materials until there's enough renewable energy to do that) and more recyclable energy (you know, diverting cow farts, dog urine and the mother-in-law back into the National Grid).
But facetiousness aside, I go back to the film Gattaca. Science Fiction has a habit of being proven right eventually as ideas become self-fulfilling prophecies. Who's to say we can't convert our cars as manufacturers find a way of designing engines to fit existing and classic motors. It might be the case that raw materials become so scarce, that that is what we'll be forced to do.
Imagine not being able to source an electric engine off the shelf for your beloved Citroen DS, XM or Xantia, but instead being able to visit the various online "Geek Factors" who have "nano engineers" capable of designing bespoke engines that are then manufactured by nanobots or nanoplants in less than a day ready for your pickup and electro-mechanical conversion.
Here's a youtube video on nano manufacturing: Click on
Nano Factory
It might not happen as quickly as the some proponents suggest (2020), but it'll happen eventually. When I was doing engineering at Uni many years ago, this stuff was being developed then. Only a matter of time before the development and execution improves exponentially.
¹To Ls in the spelling of Willson apparently