have you pulled the cyl head and looked at the walls . if ok why not just soak the pistons with some atf or marvel oil for a few days the try to rotate the flywheel or…used a wood peg and persuation to alternately give it a few wacks on each cyl.
resoak and try in a few more days
my 9622a 9.5 freed up nicely and has been running great for several years now
run some engine cleaner / decarb and run some 25/1 mix
it may be all it needs .. see utubes change the impeller gearcase oil etc…
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