You have it looking really nice. I have 2 of those mowers with Briggs motors, a 3HP self propelled and a 4HP commercial model, very heavy mowers with those thick steel decks. Both have been used every year during grass cutting season since the 80s. .I was worried alil paint over spray may have jammed me up but cleaning it off the magnets and still nothing??? I mean it's an old briggs and stratton as long as the rod isn't poking out the side if the case it should run.
The engines are original, never been rebuilt, but the 3HP is getting tired.
One of mine got wet a long time ago (somebody left it out during a bad t-storm) and it wouldn't spark. I parked it in the sun for a couple weeks to dry out and the spark issue went away.