Yardman riding lawn mower 13AC76LF055 wont start

Not sure if your model number is like 13AX614G701 It is likely the solenoid is not working, This is the component that would start the mover. See this diagram.
Replacement Solenoid 925 1426A for YARDMAN Lawn Garden Yard Farm...

I suggest that you should remove deck, turn upside down and place straight edge or tight string line over blades from outer edges of deck frame. Use the guide strips to get even measurement. Rotate blades and measure distance from straight edge to tips at both sides. they should be the same. If there is difference from one side to the other, either the blades are bent, or if each blade is same at each end but not aligned in centre (difference in height) then the spindles need packing to make them even. Check that bearings are not loose. If you do not understand this best to take it to service centre.

Outdoor Power Equipment Parts Use the Part Finder Tool from MTD
Hope this helps

Check your bladles , for turning freely.

You haven't given enough information of the problem. The blades themselves wont turn when you try to spin them by hand or they wont turn when you engage them with the lever or the switch?

Use the MTD part finder website and get the diagrams as you are seaching for the parts.

http://www.mtdparts.com/webapp/wcs/stores/servlet/ARIPartFinderView?storeId=10101&catalogId=19502&langId=-1#/MTD_Merged_Data/13637-8_(1988)/Lawn/010103700-A/0101037000001

check the belt tensioner

A 5/8 deep-well or 5/8 spark plug socket

Most common size is 19mm

YARDMAN BEATLE WILL NOT START