If someone could devise a " water jacket " which could surround a J-pipe you
might have a hope of pulling this off. This would be the technically challenging part of your project. After that it is heater hose, a recirculating pump, a heater core, a fan and some duct work. What the whole thing would depend on would be the rate of heat transfer from the hot exhaust pipe to the anti-freeze in the water jacket. Remember that the transfer of heat in the water cooled engine is next to the cylinder walls - a place vastly hotter than the exhaust pipe. You might get all of this together and discover that you get luke warm heat out of your efforts. Then of course all of that fragile structure is operating on a shaking air cooled engine inches away from the road. Given that the walls of the water jacket have to be fairly thick, like the engine block walls of a water cooled engine you would have to shoe-horn all of that into a crowded area. I'm no engineer but I would see the challenges outweighing the potential for success - otherwise someone would have tried it already. Just my opinion. Dan (MY72BUG)
I'd rather have a partial bottle in front of me than a partial frontal lobotomy.