Whatever works, Nathan. On flattish surfaces I like to use a scraper because it is fast, but your rust looks shallow in the picture, and I'd rather not remove more of the plating (if there is any) than necessary. Hence a hard plastic pot-scrubber would be great if it works. If it doesn't, steel wool. If it turns out quite a bit of the plating is gone anyway, then some kind of steel scraper is quick and effective.
In case it is useful your engine is 28 cubic inches displacement (460 cc) and was made in December 1988. AFAIK those larger engines were still made in the US, and with cast iron cylinder liners. If so it is probably a good one. So far I don't see why your mower repairer wasn't keen to nurture and cherish it. Maybe it has some dreadful communicable disease than only he knows about?
