The three alternatives to stock VW/Speedster replica heat are:
1. No heat at all (because we sometimes don't install the heater units on the exhaust pipes) and
2. Using the stock or modified heater units on the engines or;
3. Restoring an old (pre-1960 - 1980) VW-style gasoline-powered heater. These things put out GOBS of heat but only had two operational modes: Scorch the top of your knees right below the heat outlet or..... Off.
The originals were not automatic at all and required a lot of operator intervention, turning the switch on and off to control cockpit heat level. I've modified mine such that I can dial the heat output level up and down with a dash knob, but AFAIK mine is the only one that does that - more modern versions have this feature as well.
Two photos: The installed heater in the nose of the car and the dash controller under the car radio. It's a lot more involved than the engine heater boxes: