My gut feeling is that it's not economically feasible.

You are probably seeing the exhaust valve moving.

What's the engine Brand, Model type/spec.
IF it's a "Craftsman" (Tecumseh), what's the 143.xxxxxx number on the engine.

Bill, looking at the picture the intake elbow and mentioning 143 tells me it is a Tecumseh.
If you didn't run it since the discovery, everything inside must be turning dry. Go to the handle and push up and down a few times so the oil can splash around then try to start it. it might squeal for a split second and go away as the oil circulates.

All could be true or it could be the flywheel brake dragging....

@ bill kapaun - While I know that it would cost more to rebuild the engine than it would to buy a new mower, I'm just wondering if a new mower is going to last for 29 plus years. Another concern of mine is that the newer mowers have the smaller drive shafts which are prone to bending or breaking away at the key if the blade hits something hard.

After taking a second look I think you're right about it being the exhaust valve that I'm seeing through the spark plug hole.

The number on the engine is 143.344022.

@ theoutsider - I added oil and was able to mow the back lawn with no problems, so hopefully the damage caused by running it without oil was minimal. I can't hear the metal-on-metal sound when it's running, only when the blade is coming to a stop or if I turn it by hand.

@ tomplum - I'm pretty sure this mower doesn't have a flywheel brake (or do all mowers have them?) but I will definitely take another look to see if that's what's causing the noise.

Knowing my parents, they're probably going to want to buy a new lawn mower when the time comes, as opposed to being without a lawn mower for however long it would take me to rebuild it (there's no off season, they use the mower every month of the year).

If I ever have the opportunity (i.e., rainy weather or parents out of town for a few days) I'm going to remove the cylinder head and take a look at the piston and cylinder to see what kind of condition they're in.

Thanks to everyone for your suggestions and comments.

UPDATE: The noise is coming from the flywheel brake! I removed the spark plug and rotated the flywheel by hand while slowly pulling on the safety shutoff lever on the mower handle. When the lever reaches about half way, or a little less, the sound goes away. Tomorrow I'm going to attempt to adjust the cable and if all goes well the noise will be gone.

Thank you tomplum for pointing out that the noise could be coming from the flywheel brake. And thanks again to bill kapaun and the outsider for your helpful suggestions and advice.

There must have been some oil in the mower engine sump. It might have been low, but there was still some in there. Had you run the mower with a dry sump, the engine would be destroyed in less than one minute. It sounds like this mower has a few years of service life left.

Your engine is a Tecumseh TVS90-43213D

@ ericwi - Yeah, I agree that there must have been just enough oil in it to keep it from grinding to a halt. If I hadn't checked the oil when I did it wouldn't have lasted much longer.

@ bill kapaun - Thanks for the engine number info. I couldn't find the engine number in my 1994 Chilton book, their engine number listing for Tecumseh jumps from the 143.200's to the 143.500's.

Cool. Hope it works out. For future reference, the safety standards requiring the blade to stop in 3 seconds as the operator left the operating zone started on the consumer models in 1983. So if it has the original engine,- it would have the flywheel brake or blade clutch below.