2003 mazda protege es vibration. metal attached to motor mount is rotting

I took my car to a mechanic the other day and they told me to sell the car. or scrap it. I recently changed the bottom motor mount on my car and ever since. My car has been vibrated terribly at idle. When i took it in they told me that the metal the driver side motor mount is attached to is rusted out. He brought me over to the car and i looked at it underneath. So the metal as part of the frame splitting at the bottom. and seperating the binded together metal. Trying to figure out what to do and what would need to be done to fix it. or if it is worth fixing. They said mechanically its running great.