Ok, my 1303 has very rotten heat exchangers and as they are very expensive to replace so I have bought a couple of J tubes.
Now, obviously, I lose the heating and demisting so am thinking about either a couple of 12v electric heaters from Amazon/Ebay (car has alternator rather than dynamo) and recommendations for such, or the 2kw diesel type heaters or, making a water/exhaust heat exchanger plumbed into front for windscreen demist and heating
I quite fancy making the j tubes into exhaust to water heat exchangers so will need a water pump (electric Davies Craig), a fan, heater matrix (maybe two, one for waste heat?), pipework, water divert (from classic mini?).
Having completely rebuilt two beetles. I recommend you bite the bullet and replace the sills/heater channels with good quality replacements. I know
they are kind of expensive at about £200 a pair but with rusty ones the car is literally a deathtrap, it will fold in a crash.
As for the standard alternator is weedy and won't support an electric heater worth fitting. Although many EV cars like first gen Leafs have them.
But tbh it's just skirting round a much more serious issue. Good heat exchangers and good non leaky heater channels are actually quite good even in winter.