I've found that mole grips slip, making the problem worse.
The stud locks in place due to a build-up of corrosion around the point at which the stud enters the head, so the dremel removal idea works well.
Sprays really don't penetrate into the thread, in my experience- if you get it out have a look at where the spray has reached- it won't have gone through to the thread in the head.
Welding a nut on will work if you weld it securely enough, and get a spanner on the nut while it's still hot.
Another option is to file two flats onto the stud before you get a tool onto it. As long as there's enough showing, and you can file nice and square, this gives better traction for whatever you can clamp onto it.
I wonder if a nut splitter could be persuaded to clamp on? This will give a lot better grip than moley's.
But access is a problem, whichever head it is.
If you still have a bit of stud protruding, and it's the rear head, you might be better off removing the head.
Hear me out on this one.....
Trying to drill a flush stud out is difficult, as you'll have to drill freehand, at an awkward angle, with all sorts of other parts in the way. The drill bit would much rather drill into soft aluminium than steel stud, so unless you're spot on with the angle and get it centred it'll run off, possibly leaving bits of stud still in place, which will make it even harder to drill oversize and re-tap.
So the best solution is to remove the stud, and the most effective way I've found to do that is when the head has to come off anyway- typically when a cct has failed and you need to replace valves in any case- and in those cases I've found that removing the head, placing the protruding part of the stud in a vice and simply revolving the head in your hands has shifted the stud easily every time.
The stud won't slip when held in vice jaws.
And then all you need to do is fit a new cylinder head gasket refit and retime the cam chain, and you might as well check valve clearance shims while you're there looking at them. 1/2 day for the rear cylinder, 1 day for the front as it's fiddlier.
Now all of this sounds like a lot of work, but compared to drilling it out with the chances of that going wrong, might be easier in the long run. At least you'll know exactly what work needs doing, and have a very good chance that nothing else will go wrong.
I'll add that the rear head is infinitely simpler to get off and back on again than the front one, as lots less needs to be taken off to get to it.
Last tip, for everyone to consider, is to use copperslip liberally on the nut and stud every time you disturb them, and to consider re-doing it as an annual service item.....(of course if you snap one trying to undo it so that you can copperslip it and do it up again, you're back at square one...

)
It's not falling off, it's an upgrade opportunity.