What is wrong with your 650 motor? I rebuilt mine, and it turned out to be easier than I thought. Might save yourself a lot of money by just rebuilding what you have (depending on what is wrong).
-2011 Polaris RZR-4 800 (SOLD)
-1999 Polaris Sportsman 500 ATV
-2006 AC Prowler 650XT (SOLD)-- updated bevel gears, updated oil pump driven gear, 4 wheel disc brakes, line lock E-brake, Papa Slap clutch, Xentec HID's, Dynatek CDI, Warn skid plates, Intellitronix gauges, 2010 intake/cvt duct work and dust shield, RZR seats, Bestop folding windshield, full cab kit, rear cage and rear seats (4 seater)