normally those symptoms mean either a broken crankshaft, connecting rod or both. You would have to take it apart, document the disassembly for reference, and then price the defective parts. You'll need a service manual. You might be able to find good used parts to rebuild it. That said, it is probably less costly to find a replacement engine that is new, or a good core for parts/to rebuild. If the connecting rod broke and hit the cylinder wall then the case is likely not worth using. But if all you need is a crankshaft and rods you might be able to find good used parts. Myself, I would just replace the engine with a known good used or new one. There are salvage yards for mowers you can see if they have a good used engine. There are also engine wholesalers for new ones.