Really difficult to say. It could be your internal bearings on the big end. Which is not very good news.
If you look at how a 2 stroke motor works it is crucial that you have right oil mix. No oil even for a very short time means a seizure of some sort. Fitting a new oil pump ok but did you bleed it?
I would pre mix fuel in tank and disconnect the pump. Then try to start but it does sound to me you have internal problems. Possible the piston is at fault but it is a case of replacing and see. Good Luck!!
roger
May 2009