Oh....that poor Beta.
You will need to get the exhaust sealed up before it will run properly again. It's not the dent, but the leak that is causing your bike to run crappy.
You can try pulling it off, putting a new gasket in and silicon the hell out of it with high temp
silicone just to verify the problem. It may even fix it for a while or until you want to buy new parts.
At least you were riding!!!