Polaris had mechanical reverse as an option on that sled. You may be able to find the parts to change yours at a salvage yard or on eBay. I have not seen any after-market reverse kits for any Polaris sleds... Polaris is pretty much it.
Keep your eye out for a donor sled or find one someone is parting out. It may take a while, but they are out there!
2007 RMK 700, 2008 RMK 600, 1995 AC Prowler 2-up, 1980? AC Cheetah