I have an '81 500D and would like to know if there are any power add ons available for this v-twin
If you are meaning more HP they were pretty much designed to the max in factory trim, matter of fact adding things that weren't factory may move the torque curve a bit but gain you nothing unless you're willing to do some serious work. If it doesn't seem right to you get your carbs done by Larry (or use his book to the letter if you're good at it) and it will probably wake right up to your liking.
These are basically (conservatively rated at) 50 HP, 10K RPM, 10:1 compression bikes normally weighing in around 500 lbs - they are not a 27K redline Yamaha pocket rocket made out of aluminum and plastic. When in proper tune they'll run like a bat our of heck easily pulling the front off the ground when shifting to 2nd.
If that isn't fast enough for you then you need an old Kawasaki KZ1000 with the supercharger on it or a Suzuki Haybusa but the insurance will kill you.
Start by making sure everything is 100%, we've got a nice check list pinned to the top of the General forum.