Harveybiker

Hi all I have purchased a 1990 dt50mx and having upgraded to a larger carburettor, air filter and full performance exhaust. The previous owner had said that he put a 60cc bore kit in it 2000 miles ago. However whilst I was testing it, it was lagging power and felt like it was being held back. Not long after that it stopped and I presume siezed. But not fully as I could still push it and the piston could still come up and down. I was woundering if I should put a 70cc big bore kit on it? I was also wondering what I should do with the oil pump which mixes the fuel with the 2 stroke mix. Should I keep it or just mix oil in the tank when I fill it up? As this could have been a reason for the sieze? Also could there have been any engine problems that would have caused this? The piston did not have a hole in it, but had loads of carbon and some small chips out of it. I just need some advice and replies will be appreciated. I just want to get it back running.