tap it to the next size screw?

no idea what size it is though :jesus

You would want to make sure no filing go into the dizzy though.
Use lots of grease.

You may be able to run a metric tap into the hole and remake the thread - it might not be too chewed to need to go up to the next size (is it?)

I imagine it's probably an M3 or M4 or thereabouts. Buying a metric tap and die set is a great investment - it's amazing how many times you will use it. Just cleaning up the threads on your good nuts and bolts is easy and well worth-while.

Here's a typical set - 16 pieces for $140. Your local engineering tool shop will have something similar:

http://www.justtools.com.au/prod2079.htm 

Yeah, weld a nut on with your arc and run the appropriate bolt into it.

I'm hoping you meant wrecked the screw head,
which is just "use vise grips to remove and replace", but that would be too easy.
On the positive side : the dizzy could probably do with a full overhaul,
Pull it all apart and regrease, fix the screw on the bench