have a 12.5 hp ohv tecumseh engine. The exhause valve seat has become slightly dislodged on one side, causing the engine to lose compression. Wondering if it can be replaced, driven back in place. or if a new head is needed.

Yes it can be knocked right back in, evenly hit it back in. It may or may not stay in.

I had the same thing happen to my preessure washer motor. I placed a 1" ball bearing in mine and then tapped that to try to even the pressure of the hammer out. After 20 minutes of running the seat came out again. I knocked it back in and then took a punch and peened over a little bit of the aluminum in 8 symetrical spots and the seat hasn't moved all summer!