no air filter won't cause a no-start situation.. i guess some gravel got sucked up into the engine.. probably more than the one pebble you discovered lodged in the reed valve.
If it's got good spark and fuel is getting into the combustion chamber, the only other thing that could be missing that would prevent the engine from firing is compression. So, check compression.
If you can't check compression with the proper tool, check everything that could possibly be leaking compression.. that means everything above the rings..
Check for nice flat head and cylinder surfaces and a good gasket. Check head bolt torque. Check the plug .. make sure its in there snug. Check (if it has one) the decompression valve cable adjustment and the valve itself. See if some carbon (or a pebble is stuck under it :) keeping it open.
Check the rings by squirting some oil around the piston. This should increase compression if they are so badly worn out that they are leaking bad. Remove the cylinder and check see if the rings are carboned up and stuck in their grooves.
Look for a hole in the piston even. If a bunch of gravel got in there while it was running, who knows what happened..