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The pipe mounting points can put stress on the pipe alignment and pull the header out of alignment in the head.
Try this:
Remove the silencer and both (all) the front pipe mounting bolts.
Loosen the bolts holding the rubber bushes for the front pipe mounts so that they are clear of the pipe.
Remove the front pipe, clean the pipe to manifold surfaces, fit new original o rings and seal with gasket gobbo.

With just the springs holding the front pipe in place, check that it is sitting square onto the manifold and doesn't rock when you push it side to side. New springs are cheap enough so change them anyway. 10 year old springs will have given by now.

Line up the front pipe mountings and tighten them finger tight. Keep checking that the pipe stays square in the head.
Add the silencer and tighten the bolts from the front, working back to the silencer.

If you can do all that without the pipe shifting in the head, all should be good. If you can't, you should easily see where the pipe is bent and you can work at pulling it back into shape.
I had to do it with a 125 EXC as the pipes are pretty flimsy. I used an old hammer shaft in a vice and used that to bend the pipe against. Plenty of heat and steady pressure will do it. Don't jerk at the pipe or you can easily bugger it.

Good luck.
 

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It's too long since I had a 125 EXC to remember correctly but the mounting on the pipe looks bent at an unusual angle.

It wouldn't surprise me if a former owner ran it with knackered o rings.
 
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