Similar thing happened to me. I inserted a pair of needlenose pliers into the hole and opened them up, using the backside of the pliers to bear against the busted threads. Took a couple tries, but it worked.
BTW how tight should those things be, no matter how loose I make mine, they'll always hella tight when I go to adjust my clearance?
-Tim
I put these in the same category as things with crush washers, like spark plugs, exhaust flanges, drain plugs etc.
I would HAND tighten tappet covers with a glove. Run the bike and if one leaks, skosh it up a tad. if yours are tightening themselves up use some anti-seize, even though its not called for in this application.
Exhausts, drains, tighten with a short wrench till you feel the crush washer give, then a skosh more. Run it and if it leaks, a skosh more.
PS: Oh yeah, on tappet covers, always put some lube on the Oring. At least motor oil, if not something slicker like assembly lube, maybe even a touch of grease.