Depending on how hard it was rode, and pretty much a 100% guarntee the engine needs rebuilt, i am rebuilding my engine on my 2001 550, here is where i got all myparts:http://www.kawasakipartshouse.com/oemparts/l/kaw/50a64443f870022674af6aae/2007-prior-mule-1000-kaf450-b1-parts
Taken from Wikipedia.
In 1988, the first Kawasaki MULE was introduced in the form of the MULE 1000; it featured a water-cooled 454 cc (27.70 cu in) twin-cylinder engine, which was fitted to an open-cab utility vehicle chassis.[1] The MULE 1000 also featured a continuously variable transmission (CVT),independent suspension at the front and rear and a locking rear differential.[1]