Size matters here.
The 200 Franklin is 200 cubic inches. The biggest Corvair is only 164 cubes.
Cut two jugs off it, it would only be 108 cu.in, and it would look tiny compared to the 4-cylinder Franklin.
NOTE: The Porsche-powered Mooneys weren't that great 'cause the relatively small, high-revving engine required a reduction gearbox to spin a conventional prop big enough to drag a fairly large airplane around. High revs wear any engine out sooner, and the reduction box added weight that negated the Porsche engine's lightness.