Hi,
It is possible to weld aluminum...have to use aluminum rods tho....Pot metal I don't know about but if it is like cast iron, you can weld on cast iron as well but if I recall correctly I think you have to preheat prior to welding and heat afterwards to stress relieve....
If welding is not doable where you are how about going inside the hub to see how much clearance there is on the inside and maybe having a machine shop machine some openings so that carriage bolts can be used as "studs" for the sprocket to bolt onto?
Andrew