I've never seen a bent receiver tang, but it's at least possible if the thing was used as a club or someone fell on it. Likely, it'll be the upper tang that bent, since the lower is pretty thick
What to do is a problem, and you want to be very careful since you can easily make things much worst.
First idea is to send the rifle in to Marlin and let them repair it. They're the experts and I'm sure they've seen this before.
Second choice is a GOOD local gunsmith who knows his stuff.
Probably one or the other would use a milling machine to "drill" the screw head off. I wouldn't try this with any drill press since most of them aren't accurate enough to get most of the screw and not the tang.
If the hole isn't perfectly centered the machinist can "bump" the bit over slightly.
Hopefully, with the head off the screw can be extracted if it isn't bent too badly.
If it is, the shank can be milled down until it can be extracted.
If you insist on trying it yourself, buy a Brownell's Magna-tip screw driver bit.
I'd use a .270-5, .270-6, or .270-7, depending on the screw slot.
http://www.brownells.com/.aspx/pid=417/Product/MAGNA_TIP_SUPER_SETS_trade_
First, put a drop of Kroil or another good penetrating fluid on the end of the screw. As it soaks in apply a little more.
This will lubricate the threads.
Use the fitted Magna-tip bit to unscrew the screw until it starts to get tight. Stop and apply another drop of lube and give it a minute.
Screw the screw back IN.
Then unscrew until it gets tight again, then back in.
Each time you screw it out, try to turn it JUST a little farther.
Hopefully this will allow the screw to "iron" itself out.
If it doesn't seem to get any easier... STOP. Take it to a pro who can get it out without damaging the tang threads.
When/if you get it out, use a soft copper or lead hammer to gently tap the tang back in shape.
Note that the tang may snap right off if you hammer it too hard or too close to the front.
Marlin would probably induction heat the tang to straighten it, but you can't since that would damage the receiver.
Worse case, you may have to sacrifice the stock to get the screw out.
In this case, you'd cut the stock off and away from the tangs and screw. With the wood gone you could cut the screw in half and unscrew the bottom half.
Again, my first stop would be Marlin.