The Wankel (not tittering, matron) is actually a very good motor for rotary mower. Right shape and would only need a lightweight simple gear train to drive the blade. Just Googled it and it’s been done! A firm called Sachs made and sold them in early 70s. This one has its head (side?) off to prove the rotary piston is actually there…
For larger jobs, a cut-down version of the Wright cyclone radial aircraft engine would also be brilliant (and guaranteed to annoy neighbours).