How do you repair an elbow pin?

I was about to suggest the exact same thing.

I'd heat up the joint and remove the pin entirely, if it is well and truly broken then I'd replace it with a piece of wooden dowel. Really though anything you could find that's about the same size as the pin hole would do the job as long as it's strong enough. Then I would fill the end of the pin hole with sculpt, wait for it to dry, give it a good sanding then paint. You'd have to do the sculpt and paint stage on the other arm if you wanted to even it out. Hope that makes sense bud.