Re: What mod should I get.
Ouch... once the threads are stripped, the head is pretty much done for... You could helicoil repair it, but the cost of the helicoils and the tools to install it would be more than the head is worth IMO.
Option B is to get a Puch Hi Hi head from treatland for about $40 which will help keep your engine cool way way better than the stock head, or you could get a Fred head. Fred heads cost around $70 but they're really nice looking and excellent for keeping an engine cool. there's a new head out called the Diamond head which is really nice too, it's pretty much the same as the Fred head except that it's square instead of round so it matches the engine a little closer... Any of these heads will give you better cooling ability so adding more power later is nice and safe to do.
Best options are (budgetwise)
A... Try to get a warranty return - Free, Hopefully...
B... Puch Hi Hi $40
C... Fred head or Diamond head $70 to 80
Best options (performance and cooling wise)
A... Fred head or Diamond head
B... Puch Hi Hi
C... Stock head, modified by sanding about 1mm off the combustion chamber side
The Puch and the Fred head have been tried out and the performance and temperature difference isn't really that big, but the price difference and appearance difference is there so it depends a lot on budget and your personal taste, I'm assuming the same for the new Diamond heads as they are very well built as well.