It's worth considering fixing it up; that's what we do here
The muffler bolt holes are usually fixable with Re-Coil thread inserts. Easily done if the bolt stubs aren't stuck in them, but still usually do-able if they are.
If there are no bits missing underneath to cause it, the vibration could be due to a bent blade disc, or in the worst case, bent crankshaft. It isn't at all likely to be anything higher up.
For a start, tip the mower up [spark plug uppermost], turn the engine over, and see if any wobble is visible. Look for missing bits like a broken blade, too. Report back on what you find.
I'll mention at this point, that on one of these machines, a replacement engine has to have an output shaft that's machined for
two woodruff keys, for the pulley and blade disc boss. As well as being the correct diameter and length, of course.
May not be so easy to find this; no doubt one of our industry people can comment on this issue.
