This IMO is the best, easiest and most effective solution. About £130 to buy, but after that they are cheap to run. Drier air heats up faster the drier it gets. There's no heating element in a dehumidifer but as the garage dries out, it does get warmer because the fan motor gives off heat like any other motor. I had a garage the same as yours, but had been left damp and unloved for years, there were quite a few gaps. My dehumidifier would extract 2 litres of water in 8 hours. It's amazing how dry it got even in winter.
Long story short, I kept a Honda CBF125 in there over 2 winters. They are famed for being rust magnets (cheap bike made in India) but mine was spotless. No rust whatsoever.
*Edit - I also haven't changed the filter in 7 years. It still works perfectly