Short answer, you need to replace it with a 2mm brass nipple of the same overall length.

Longer answer, you might consider how and why the nipple broke and consider replacing the spokes if they're all too short.

Rather than start fresh, I'll point you to a recent thread about this very issue.

When it's time to consider your options, don't forget to factor how much use you expect from this bike. If planning a long tour where reliability is a paramount concern, you'll want to fix it accordingly. Otherwise, "good enough for government work" is good enough.