I did try running it again before taking it apart, couldn't get it to stay running and sounded as if the exhaust had fallen off, far louder than usual?

So you think there is nothing wrong? Is it worth me changing the piston or seals at least?

Yes, of course there is something wrong. But I don't think the fuel alone caused it.
Regarding the loud noise, does it come from rear of the silencer or does it come from a leak some place close to the engine? Could it be just the silencer itself that is your problem?

Regarding new seals; yes, when you put it togehther you should use new o-rings and gaskets. If you measure the piston and ring gap you will get an idea of how worn it is, at least if you have some numbers to compare your measurements to. Unfortunately, I have no knowledge of the wear limits of the 125cc trials engines.

The loud sound is 100% coming from the exit of the exhaust, no holes or breakages