The flange broke where the one bolt goes through. So I can only bolt the toilet down with 1 bolt. Do I just bolt it down with one bolt and drive on or do I have to replace the flange?

THere are flange repair kits that have a curved piece that bridges the broken part from the bottom...

But never use just one bolt... you WILL have nothing but trouble.

Agree one bolts is asking for trouble. You can use a steel repair ring or one of these YouTube - Toilet flange repair (http://www.youtube.com/watch?v=LaVNXgK3bNc)