That extra information is useful. In theory, if the behaviour of the machine changes suddenly, something must have happened to it that caused the change. There is one thing we know about that changed: an old, coked-up engine was turned on its side, then suddenly stopped running. The muffler probably had a large amount of carbon in it, and turning the mower on its side may have caused it to shift. You may have a blocked muffler. I was recommending that you remove the muffler just in the interest of completeness, and because a blocked muffler seemed to fit the symptoms. From what you have now said, it also fits the circumstances. At a minimum, it needs to be disproved as a possibility before you go further.