I would start with what you can see is wrong.... so fix the gasket, and visually check everything. Even a slow speed collision can jar something lose. So fix the exhaust gasket, check the o2 sensor (though to me it seems unlikely that that would be the problem without a code) check all cables and sensors to make sure nothing has come lose....
Sorry about the bad luck.
Defending the reputation of the red-headed step-child of the focus world for far too long.