If you have a smaller engine (<1915 cc), you can run the stock diameter OEM heater boxes - a bilge blower assist helps flow. A larger engine will take larger diameter heat exchangers which are costly. Another alternative is to add a reconditioned or new gas heater - it provides heat that you'll have to turn off. They are more costly. In Ohio I'd think heater boxes would extend driving time and be more comfortable than NO heater.
Eberspächer was one of the original suppliers of gas heater for VW and Porsche. They made them for bugs (BN2), Bus/Campers (BN4) and the 412 (BA4?). They are available used but parts are getting hard to get. Maybe $400. New ones are also available for around $1-1.2k.