The XJ-6 we had was a Canadian model... As I remember it had a heater built into the lower radiator hose with a regular 115 volt three prong cord that could be plugged into an extension.
Perhaps the Canadian dealers or parts suppliers could locate this for you. Alternatively, an old New Hampshire trick is to leave a work light or any other 75 to 100 watt bulb burning next to the engine. Obviously, a new style compact fluorescent of LED bulb won't work. They convert almost all of their energy consumption to light, not heat.
Good Luck from New Hampshire.