You can probably grab the bent end of the threads with a vise-grip and bend it back and forth enough to break it off. Then drill a hole into the center of it, from that end. Then get the Harbor Freight "Heavy Duty Chain Breaker" tool for about ten bucks and drill out the receiving end of that so it fits over the cotter head. Now screw the pushing end of said tool down and watch to make sure the thin pin of the chain breaker goes straight into the hole you drilled. Snug the coarse threaded part down as far as possible, then tighten the fine threaded part. Magic!

I know this sounds complicated, and some of my description won't make sense until you have the tool in hand, but this technique is guaranteed to work and will not damage your crank in the least. If you want to pay shipping, I'll even lend you my Harbor Freight tool, which has already been modified (but it'll be cheaper just to order one yourself).