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You cannot weld to it. It is made of cast brass, the reason it strips the internal hex is that it is cast and the hex has to be tapered to come out of the mould - no Allen key ever really fits properly.
This is not much consolation to you though.
Aluminium will expand more than brass on heating so either get the engine really hot or use a blow torch around the outside of the plug.
Before doing that I would get a 2 or 2.5 mm drill, having centre popped it first and drill about 2mm into the head of the plug as close to the outside edge as possible. Choose the location of the indentation carefully having regard to the next step
With the engine warm or pre-heated with a blowlamp get a pin punch and drift the plug anti-clockwise at as shallow an angle to the crankcase casting as possible a) to maximise the turning moment on the plug and b) limiting the cracking stress on the crankcase casting - the shallower the angle the more crankcase material is absorbing the blow.
Heat alone might do it so worth a try first.
Good luck
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