I don’t have a gas welder, only a MIG. While it’s always great to have an excuse for a new tool, it takes practice from what I understand to be able to gas weld (especially thin sheet metal.)
If I had the tools to ensure concentricity I’d try the washer trick. I was thinking that a jewelry welder might be a possibility, but I don’t think the under $250USD models will be up to the job (they’re mainly for spot welding and have no filler rod or wire.)
I’ll see if the machine shop I take the pulley to will have any ideas. I would think I can just use the other undamaged keyway so that’s what I’ll do if I want the car on the road ASAP.
I found this batch of Lucas E-type fans on eBay. Alas, the one that matches my fan is the one that’s damaged. 
But maybe I can use one of the others.
Jaguar XKE E-Type Generator Fans - Lot of 4 Pcs | eBay
Dave