I had the same issue on my dad’s lawnmower Troy bilt 20 hp Briggs and Stratton. It ran for him but I went to use it it dead headed. I as well thought bad starter or bad battery. I did my research intake valve was out 1/8” exhaust valve out 3/8 of an inch I looked at spec adjustments called for .003 in. On intake and .005” on exhaust, no bent rods how he got lucky on this is beyond me.