My vote is for:
20" HRE 504's, just extend the crankshaft down the same amount, to retain your cutting height as before.
Some semi-knobby tires, ( anything but a french make ), oh & make sure you get them balanced static & dynamic, with the weights on both sides. Not sure about roadforce requirements ( unless you ever walk your mower down the street to the nearest bar ). The Gardenmaster GSP 9700 should serve all your balancing requirements.
And NEVER over or underinflate the tires.

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2007 C 230 Sport.