Spiral wound tube boiler inside a pipe just after the cat.
Insulated spherical reservoir that can handle high pressure.
Engine like the previous link attached to the alternator like a pulley, with an electric clutch to engage a belt if you did not generate enough steam.
Use available heat from condenser to heat passenger compartment.
A small air grinder can spin a wheel up to 100 MPH so you might want to use the die grinder to spin up a flywheel then engage the field to place a load, and pulse and generate the system.
Get rid of cooling system operated heater core and hoses.
I doubt you can do it cheap with off the shelf parts, but if so a rotary vane pump would be your best bet. You would need at least 3 cylinders and pistons for it to be self starting.
regards
Mech
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