The carb for your engine should be Tecumseh part number 640330A. I have found this part, Tecumseh branded, on Amazon for $14.00. What was the brand and part number for the carb you got. Second, what did you set the coil gap at and each of the valve clearances? If you got a off brand carb with a jet too large and then you drilled it even larger, could also be your problem. To me I would be looking at a sticking valve or incorrect valve clearance, but with an aftermarket carb I can’t rule that out. Did your replace the head gasket when you pulled the head, I would never reuse a head gasket on an OHV engine.