My buddy tore off the rear derailleur on his mountain bike. I've been tasked with straightening the derailleur hanger.

The bike has vertical dropouts.

My plan was to verify the alignment of the dropouts first (I have the H-tools), then align the hanger.

I've discovered the axle slot has widened to 14mm as a result of the accident.

What's the best way to close the slot to the proper 10mm dimension?

As a minimum, I figure I've got to stick a derailleur mounting bolt in the hanger to keep it round should I decide to somehow leverage the dropout.

Any help would be appreciated.