The Elephant in the room regarding "classic" cars is that nobody will be driving vehicles with Internal combustion engines in 20 years time. Everything will be electric/battery/hydrogen by then.
A couple more smallish advances eg. ramp up the infrastructure (charging points) and make a few improvements to battery life ....and its curtains for fossil fuel engines.
So your future classics could be worthless .. it will probably be illegal to drive them. (or prohibitively expensive to pay for a special carbon emissions license $$$$)