The mower tried to eat a protruding root, emitted a horrible crash, and ran poorly until I shut it off, after which it refused to restart. Hoping against hope that the flywheel’s aluminum key had sheared, I pulled the cover, removed the starter, and found:
Alas, the key is in fine shape. I made the two diagonal scratches to confirm it really is aluminum.
After letting the mower sit for a day, it started and ran briefly, blatted a giant backfire that probably startled the neighborhood (because I had the exhaust aimed into the garage, which served as a wonderful resonator), died a sudden death, then made clanking sounds whenever I pulled the rope. Something is definitely broken inside, but I suspect diagnosing & fixing it will require more time and money than is justified.
I no longer form deep emotional attachments to lawn mowers, so I ordered a similar one online and the local Sears had it ready for pickup in an hour.
If I had to pull the flywheel, I’d tap the two obvious holes (one behind the shaft in the picture) and gimmick up a puller with two matching screws around a central bolt that does the heavy lifting; I can’t justify the Special Service Tool I’m sure it requires.
The old mower lasted an hour at the foot of the driveway with a “FREE – Engine probably severely broken” sign affixed to its handle; both parties got a great deal on that transaction!