Briggs and Stratton model 12S902 won't start

Hi guys. I'm new to this forum. I tinker with Lawn Boys but my neighbor brought me a Troybilt power sprayer with a B&S model 12S902-0143 B1 engine.
He said the engine was surging up and down quite a bit. He said he was moving the linkage on top of the carburetor and it backfired and quit, never to start again. I cleaned the carb and and reinstalled it. Cleaned the rust off of the flywheel and coil. Checked the gap at .0010 of an inch. Made sure the flywheel key was not sheared or damaged. Checked for spark and had good spark. Cleaned out the gas tank and put fresh gas in. Put it all back together and it wouldn't fire a lick. I hooked my garden hose up to it also. So I took the ohv cover off and the valves move in and out some but I'm not sure how much they are supposed to move when I pull the rope. It wouldn't even fire with starting fluid. I thought that was weird. So I borrowed a compression tester and when I gave it some pulls it didn't show any pressure at all. The needle didn't move at all. I don't every remember seeing that before. The carburetor wasn't on when I checked compression though. I don't know if that would make a difference or not. It feels like there is a little compression but nothing like the 2-strokes that I have known. I also just read that some of these B&S engines have plastic cams that could go bad. Any help, thoughts or ideas would be greatly appreciated.
Thank you.
Jerry

Have you confirmed that the piston is moving up and down, and hasn't broken the rod. If that checks out OK, Check valve clearances to make sure you have the required clearances. I suspect you have a valve issue, but if the above check out, then may be time to pull the head.

Thanks for the reply. Yes the piston is moving up and down ok or actually in and out. I stuck a pencil, eraser side in and pulled the starter rope and that works fine. Do you know what the valves should be set at?
Thanks

..004-.006 comes to mind. When you look at the end of the valves do they look even, making sure they both stick out of the head about the same distance. I have seen the valve seats out of various engines and will act just like it is in your case.