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
You might want to try calling www.Alssnowmobile.com They sell a TON of used parts and are probably in the know about what fits what... I looked on the Polaris website , but they don't even show the reverse style chaincase in the microfiche for your sled... you can see the other sleds that your chaincase fits, if you find one of those and it has reverse, it should work. When you're looking at the Polaris part number, scroll that window to the right and you will see a "usage" column.