could be just hardenned grease… remove the handle at the engine and spray some release agent down the handle if you can’t remove that screw . it may soften thing up for you while you rotate the handle back and forth) and you won’t damage that screw
give it a good rinse with some solvent to flush out any old grease ( varsol, turpetine, fuel oil etc…)
… reinstall
https://www.youtube.com/watch?app=desktop&v=ghYPW4-PJ8k
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