There is no M22. Just M18 and M20 and a bunch of other vertical shaft models.
I'm pretty sure the M18 and M20 have a different bore and stroke. I would not count on the jugs being the same casting, either. About the only thing that wouldn't have to be changed is the case halves.
If you want to know what is different, go to the parts lookup and compare engine part numbers for the 1872 and 2072, as the former has an M18 and the latter an M20.
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