Very nice job of building the cab!
Keep the snow off of your boots before you get in, as it will end up condensing on the windows!
If you think you can use even more heat off the engine, you might consider installing a duct fan in the heat tube in the can. It is just a small electric motor with a fan inside of at tube you would connect to the heat duct. Suction would pull even more heat in the cab. Does you ground freeze up, or do you get enough snow that it thaws and stays dig able in the winter?? Some times we get enough snow here ground heat thaws the dirt out in the middle of winter..
I have to wonder as well, if you could not find the parts for your engine to make able to charge the battery. I know in some cases the manufacturer offers those parts, but not usually inexpensively however..
Regards, Kirk