The stock chain/sprockets are a 530 (its a rating based on strength/width of the chain). You're hard pressed to find an aftermarket kit for the 530. Most of them are 520 conversions. With that said, if you do a 520 rear sprocket and chain, you have to get the 520 front sprocket as well.
Now for which to choose...
The most common change is -1/+2. This removes one tooth off the front sprocket, and adds 2 to the rear. Adding 3 to the rear is the same as subtracting 1 from the front (so a -0/+5 is the same as -1/+2). You can get any sized sprocket you want, but the further you go from stock gearing, the more it will jack with your speedometer/odometer/top speed, etc. Find a speedohealer...they're a VERY good idea if you change your gearing. You'll be good with the stock length chain. Good luck!
Now for which to choose...
The most common change is -1/+2. This removes one tooth off the front sprocket, and adds 2 to the rear. Adding 3 to the rear is the same as subtracting 1 from the front (so a -0/+5 is the same as -1/+2). You can get any sized sprocket you want, but the further you go from stock gearing, the more it will jack with your speedometer/odometer/top speed, etc. Find a speedohealer...they're a VERY good idea if you change your gearing. You'll be good with the stock length chain. Good luck!