Does the HP seat leak? If not, leave it alone! Getting a kit isn't a bad idea for the future, but if this ain't broke..... You should replace the rubber can to valve body gasket, put in a new duckbill, or better yet, a DBE, and the main diaphragm. If your lever adjuster screw and nut are stuck, you can buy a new stainless allen setscrew and a new stainless nut on line or at most hardware stores.
If you have a leak, take the bottom can off and the diaphragm out. Put it on the tank and check for a leak. If it's hissing, check your tank o-ring, and any other possible leak site. Just because you hear a leak, it doesn't mean it's the HP seat. Spray it w/ soapy water.
If it doesn't leak, your diaphragm lever adjustment is the most likely culprit.