Check chain has the correct amount of play, rear wheel is aligned properly and bearings are good. Doubt it's exhaust related, many many people put after-market systems on there's (Inc me) and haven't had a problem like this. I would make sure your 2 stroke oil pump is working though.

i got off and it was fine for about two minutes then done the same, back will free wheels fine i wondered if it heat sized, or as the new exhaust draws more petrol the 2 stroke pump isnt putting enough oil in the fuel

i just looked at the piston and it is scored so this must be to do with the 2 stroke oil?

It has nothing at all to do with the oil or water ,the wheel bearing as most likely jammed .

Could be your mixture is now too lean so it's overheating and seizing. Seems unlikely it's not linked to the new exhaust.

I just got my new exhaust so i fitted it and took it for a ride got up to about 50mph and the back wheel locked. i have checked the oil and water level so what could this be? any ideas

thanks