How do you remove a screw from the valve cover that broke off?
If there is still a little stump left (e.g. you were able to
remove the cover), you can use a metal saw and saw a small groove
on the middle top of the remaining thread. Then use a screw driver
to remove the stump. Most likely you will need to use WD40 first to
loosen the screw. You might need to use an Impact Screw Driver to
get it out.
If nothing is left, you can try to drill a small hole in the
middle of the remaining thread. You then need to either get a
special reverse thread tool, or a reverse thread screw and try that
to remove the remaining thread.
As last resort, you can drill it out and re-thread the hole.
Start with the smallest drill bit first and remove as little as you
can each time by increasing the drill bit size.