In reply to Sharp:
It's probably cast iron, so will drill easily.
I'd drill through by some glass of ebay and refit.
Not sure helicoiling would cope with the heat / cold cycle when there isn't a good lump of metal around it.
A piece of sheet steel will see you through the winter, but you'll loose the lovely glow and the radiated heat, though it would be considerably cheaper
welding isn't really and option, you need special rods and a lot of skill, it usually still cracks so you have to grind and refill, nightmare and time consuming.
Good Luck