I ran my HRR216 mower without oil until it seized (don't even ask...).

I got it unseized and started, and it seized again. Unseized it again (more easily this time) and now have it running.

I've run it until hot and then changed the oil twice now. The oil is thin and black after 15 minutes of running. Also, when hot and under stress (higher grass, for example, or self-propelling), it rattles pretty loudly.

Thought I'd start with the flywheel. Took the cover off and spun the wheel by hand, which seems to have a hitch. With the flywheel seated, I can wobble it slightly side to side. It spins freely, but I hear a clank under the deck when I spin it. Haven't taken the flywheel off, as I don't have a puller, but I will get one if it may offer a solution.

Could the key be bad and the mower still run (and also produce rattling sound)? Could it be something else related to the flywheel and still have the above symptoms?

Things I'm also considering are: cracked head gasket; loose pin at the piston; loose bolt at the connecting rod; cracked connecting rod. Thoughts on those? I'm willing to open it up and investigate, but anything else I should check before doing so?

Thanks.