Hi 1376575 If the machine runs, while it is running; spray a mix of soapy water around all the gaskets/seals and see if there are bubbles. The professional method is by doing a 2 Stroke Leak-Down Test. A home version of that can be accomplished as well. You will need to block off the intake and exhaust ports. I have used thick little rubber blocks for this and the muffler and carb mounting bolts with large flat washers on them to seal. You will then need to introduce compressed air into the engine through the spark plug hole. 5 to 6 psi is plenty. You will then hear or notice a pressure drop if you have any leaks and have a pressure gauge attached. You can spray soapy water or what have you around the gaskets and seals to check for those leaks.
Potential leaks will likely be around the crankcase seal and the cylinder base gasket. The crankshaft seals have been known to dry out and/or warp and leak as well.