Should be 090000 series but the engine numbers will be on the metal shroud under the gas tank. I think it on a metal tab over the muffler on this one.
Could be just carbon on the piston. Valve clearances do close up on these. As far as jumping valve timing I have yet to see one do this as it is metal to metal gear train. If if not just needing decoking or valve problem then you need see inside the crankcase for problems.
Either if it is late 80's it had a good life and may just wants to retire.
I do still work on some Briggs engines from the 60's and 70's here but most of them are horizontals.
Note: metal knocking sound could also be just the cutter hitting the mower body too. If this is the case then the flywheel could be partially sheared.
Just a few thoughts though as I prefer to put my hands on these problems.
