Remove the spark plugs and check out the cylinders. If you don't see any hay or other detritus try 2oz of Marvel Mystery Oil per cylinder.
My own experience along these lines relates to a Gordon Smith Compressor. The device had been operable when it was put under a barn to store it so I could build my house.
Six months later I recover the compressor, tow it home, and try to turn it over for a start - STUCK.
Removing the head I found that as the engine had stopped the No. 4 cylinder exhaust valve was open, and mice had crawled up the exhaust pipe (it had a muffler), squeezed past the open valve - and set up shop for the winter.
Mouse urine is highly corrosive.
The engine already had some "air miles" on it (compressor) so I elected to have the block bored - and oh, while we're at it clean up the crank and rods and re-set the engine.
If on opening your plugs you find ANY vegetable matter, your outlook and wallet might be the same.
Joe k