B&S 6.75 126M02-1031-F1 suddenly backfires, will not start

In July, 2013, I replaced a blown B&S on my Toro SR 20055 with a Briggs and Stratton 126M02-1031-F1 190cc 6.75 Engine. Have not had a single problem, at all, until last weekend. I used it, let it sit for an hour or so, then tipped it slightly onto the side (gas tank was virtually empty) to scrape off some gunk that had built up on the kickers. Added gas. Went to start it back up, and nothing. Pulled a dozen or so times, and nothing. Not even a sputter. Then, bam ... sounded like a *very* loud gunshot. Not your average backfire. A dozen more pulls, and the same thing. Seems to be coming from the muffler, not the carb. I didn't hit anything. It hasn't been banged around. Already changed the plug a few months back. I can't understand why it would suddenly die like this after only being tipped on an angle?? I tried today, a week later, and the same thing. Even tried starter fluid. no luck.
I haven't checked the flywheel key yet. Any other recommendations? Sticky valve perhaps? Aside from ripping apart the engine, any way to remedy that?
Thanks in advance!!

B&S 6.75 126M02-1031-F1 suddenly backfires, will not start