you can get kimpex piston kits for about 44 a piece and a winderosa complete gasket set for 24. SPI pistons are also good, but i think they cost a little bit more. as for performance, you could trim your base gaskets and other gaskets so they don't inhibit air flow. when I had one of my 503's tore down i cleaned up all the intake, transfer and exhaust ports. i didn't get to ride it much last year so i don't know if it made much of a difference. maybe check your clutches out for wear, my sled only had a couple thousand miles on it and both clutches were really worn and weren't working properly.