That is one of the three long bolts that anchor the engine to the frame:
http://www.ronayers.com/oemparts/a/k...1/engine-mount
Actually, it keeps the frame from bending, as the engine is a load bearing element of the frame.
You need Kawasaki part 92015-1180, which is a flanged M10x1.25 mm hexagonal nut (external diameter of bolt is 10mm with a pitch of 1.25 mm).
Use a 14mm socket and apply a torque value of 24 ft-lbs.
Although OEM is the correct grade, any temporary nut will keep the bolt from damaging its thread due to vibration and backing out.
I have found good quality hardware at PepBoys.
The bolt is what does the shearing resistance, while the nut keeps it in place while reduces overall vibrations.
If I remember correctly, there is room for two nuts there.
Check the one for the lower bolt as well.