riding lawnmower wont crank

I have a 2007 yardman 46" riding lawn Mower mtd, Briggs and Stratton engine. Mower had been sitting since last year. The mower will not crank. I replaced the battery. Wouldn't crank so then I replaced solenoid and small 20 amp fuse. Wouldn't crank. Then tested starter and replaced with new starter. Wouldn't crank but this time at least the starter spins but does not engage the flywheel to turn the moter. The starter is the right size but when I turn the key the starter seems to just spin as not having enough power to turn the engine. After replacing the battery, fuse, solenoid, starter I don't know where to go from here. Any help would be appreciated. Thanks
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