I have rebuilt quite a few engines . I can tell you often it is more economical just to buy a new one if heavy damage has occured or its worn out.sometimes it pays if only one or two parts need replacing.if nothing else ,its an education. you must pay very close attention to how things are aligned& placed so it goes back EXACTLY the same way. normally, I would not recommend disassembly of the piston& liner for crash damaged motors . flushing the engine with the head removed , backplate removed& carb removed usually works. 3 in one oil works for assembly oil