Rounded Off Headst bolt

I have no idea how you managed to do this, it doesn't need to be particularly tight.  Undo the bolts that hold the stem onto the steerer. Hold the front wheel down, pull the bars up and turn them anti-clockwise.  ease of the pressure turn them back and repeat.  Hopefully the bolt shoud loosen.  Once you've got a new bolt, make sure you've read up on how it all works before you put it back together.  For the avoidance of doubt, it is the stem bolts that do the work, not the bolt in the cap.

Drill the head off carefully. Remove the top cap, should allow access to the rest of the bolt. If it isn't seized cutting a simple slot should mean it can be undone simply enough with a flat head screwdriver, or if it's loose enough by hand or a pair of pliers (but be careful not to chew it up even more if it is at all tight).

Not familiar with Zipp headsets, whether it is an insert expander or a star nut, but having removed the top cap you should be able to see. An expander should release with the tension off the bolt, a star nut may need forcing out.

Any engineering supplies place should be able to supply the correct bolt, I'd get one in stainless and make sure you assemble it with some anti-seize.

It does beg the question why that bolt should be so badly damaged in the first place? You do realise that all you do is nip it up to remove play in the headset before clamping the stem to hold that tension? I could then be loosened and it will make no difference, it merely holds the cap on so doesn't need any tightening above being nipped up. Presumably it got mashed by trying to undo it? If that is the case I would suggest it had seized due to not having any coppaslip or the like applied to it before installation. Headset and top cap bolts can get covered in salty sweat so anti-seize of some sort is a must.

PP

I'm assuming you have fixed the problem by now, but if not, can ask as a general question.... is this something that is experienced regulary on bikes? 

In the main there are two reasons why a fastener head becomes damaged, when its tightened up in the first place and when its being undone disregardless of the quality of the fastener.

I'd try a the correct size torx bit and a bit of elastic band. Put the elastic band over the bolt head, push the torx drive in and then try and undo it. If you are lucky, the torx drive will get enough purchase to undo.

Good luck and hope you get it out successfully whatever method works.

You might be able to cut a slot with a dremel tool which will allow you to use a flat bladed screwdriver to unscrew it.

Would need the right sized dremel attachment and a bit of care to try this.

One trick for this is to gently hammer an oversizedTorx key into the hex recess. You may get enough purchase on the bolt to turn it. It shouldn't be very tight in the first place.

I'd be very wary where bearings or star nuts are involved as here. Also.that bolt looks a bit far gone for anything but a bike shop visit.

...edit....Just googled  Zipp headsets. It looks like they use T25 torx keys, so ignore my suggestion above.( It does work with rounded hex (Allen) bolts.) Have you got the correct T25 key?  There are similar looking keys available that are subtly different...I got a whole set with a cheap screwdriver kit once.. Good luck.

Get the drill and stud removers out...

By the way, Allen bolts are hex bolts, just a trade name for them.