Bill,
The existing fan can stay, duct can stay, install a manual damper on the intake and you can open and close it as necessary and maybe just add a heat coil to it and waaalllaaa you have what we call a make up air unit.
Gas burner while painting .. i did it all my life ..too. ... I say add a coil to the the make up air fan and your done or just move the fan to the exhaust side and add the coil on the intake.
my 2 cents.
By the way really nice job on the shop ... best of luck...
Ron
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