Repair: what to try next? Toro walk-behind w/ Tecumseh 362003D

At the start of the season my lawnmower was visibly leaking oil from the crankcase/oil-sump gasket. I just replaced the gasket and put everything back together. It started right up on the driveway, but I feel like it was louder in a way that I can't really describe. I'll try: I'd say it was running harder (not faster) than usual, a little extra percussive. As soon as I moved it over the grass (recently cut, not tall) it died - and will not restart. The pull-start moves the blade as usual, and I can hear the piston creating and exhausting pressure, but no ignition.

What might I have screwed up? What should I check next?