[CHuDWah]

There are manifold heaters for flatheads but the ones I've seen are complicated and ugly - plus manifold heaters are hard to regulate. Aftermarket places like Vintage Air sell compact hot water heaters but they're just black boxes. There usually are some vintage ones on ebay - they're just a small radiator with an electric fan but they usually have a cool art deco cover. They're mostly six volt but a twelve volt motor can be adapted with a little ingenuity. They can be recored but of course it's cheaper to get one with a good core. Otherwise, just refinish the cover as necessary. You can buy new rotary switches that look old-timey and allow you to vary fan speed - it also helps to put a shutoff valve in the water line.

For example, this one's currently on ebay - even looks like a Model A radiator: