TS-125 Exhaust blowing

I have been rebuilding a 1979 TS-125 Alpine for about 6 months. It is now almost ready to go back on the road but I am getting a lot of blowing between the exhaust and the silencer. I have taken the silencer off, cleaned up the two mating surfaces and clamped it all back up as best I can but there is still some leakage. I suspect one or the other part is now no longer properly round so I am not getting a proper seal.
Can anyone advice anything I can use to make a better seal?
Greg
Can anyone advice anything I can use to make a better seal?
Greg