Small Engines (Lawn Mowers, etc.): Briggs L-Head Problems, piston rings, starter cord

Question
I have a 13 HP 28M707 engine on a trail mower that was running perfectly until, while mowing last week, it lost power and quit running.  I knew it was bad when I pulled the starter cord, and there was no resistance.  The oil had apparently gotten low, because it was off the dipstick, but I don't think it ran completely out.  I tore the engine down and found that the connecting rod had broken right in the middle.  I bought this mower used, so I have no idea if it might have hit a stump or some other obstruction before I bought it, but somehow the connecting rod failed.  There was a considerable amount of debris clogging the cooling fins and starter shroud, so between the low oil level and the air restrictions, it may have gotten quite hot.  The crank and bearings were ok though, and the piston, rings, and bore seemed ok too.  No major scoring on any surfaces.  I bought a seal kit and a connecting rod, polished the bearing surfaces lightly, and reassembled the engine with the new rod and seals.  Now, when I try to start it, it starts but runs weakly, won't come up to speed, and it seems to be spitting a little fuel out of the carb.  I believe I have it timed right (I marked the timing gears when I took it apart and when I reassembled it with my marks aligned and rotated it through, the factory timing marks lined up perfectly.  I didn't disturb the magneto, I cleaned the carb thoroughly, and I made sure the flywheel key was in good condition and positioned properly.  The only thing that happened in the process that I'm not sure is back to original are the push rods.  They fell out during the process, and I had to guess which one was which.  They are the same part number but I had no way to measure them or be sure if I got them back in the original bores.  At this point, I'm thinking that perhaps the valves are out of adjustment or leaking, or maybe the bore and rings aren't as good as I thought they were.  I just ordered a service manual, and I plan to go through the factory rebuild procedure, but I was wondering if maybe you had some input that might save me a complete rebuild?  It was running so well before this happened, I'm having a hard time believing it needs cylinder work or a valve job, but you may have seen this happen before and know what other damage I may be missing ...

Answer
Find valve clearances here:

http://www.briggsandstratton.com/us/en/~/media/Frequently%20Asked%20Questions/En

Make sure the governor is adjusted:
http://www.briggsandstratton.com/eu/en/support/faqs/adjusting-the-governor

I'd check the valve clearances:
http://www.briggsandstratton.com/eu/en/support/faqs/servicing-the-valves

Let me know if the links help.
Eric