Do NOT use space heaters in your garage. Regardless of whether it's kerosine, propane, or electricity, space heaters are notorious for causing fires. And because they all have warnings on them saying not to leave them running unattended(either on the box, the information thta came with it, or on the heater itself), your insurance doesn't cover it, and you can't sue the company if it causes a fire.
In my dad's business, we had over 75k worth of damages that the insurance would NOT cover.
Please, pay attention to the warnings on those. They're not something to mess around with.
I'd say the best thing to do would be to run another heating duct from your house to the garage. But, that's gonna be expensive.. I dunno what else you'd do. Maybe install a small furnace in the garage itself.