You say it won't start, but does it turn over? In the past, did the starter struggle to turn the engine over? Does it try to turn over, but doesn't seem to have the power to do it? If this is happening, pull the spark plug. If it cranks Ok now, the problem could be that the valves need adjusting. Most of these more modern engines have compression release built into the cam shaft to leave the exhaust valve open slightly as the piston goes up, and then the valve closes during last part of the compression cycle. This makes the engine easier to crank. If the exhaust valve clearance to the push rod is too much the remains closed and there is too much compression for the starter. Adjust the valves precisely. This is a 4 cycle, right? I've seen this before and a new starter won't solve it. Rope pulls are also very hard under these circumstances.