Tires...I go Cheng Shin, all the way, to my experience the best moped tires they make.
Tubes...I go to Walmart and buy "BELL Self-Sealing" bicycle tubes, and I buy them 1-inch larger than the tire they go in, they work very well and will not freeze in subzero temps (handy being that I live in Michigan)....the extra rubber from buying a size large seems to help on the puncture-resistance as well.
I've only had one flat since, and that from a small panelling nail that tore eight holes in the damn thing cause it went in there and spun.
Rear Sprocket, well...you will lose a bit on the takeoff/power end, and get it back on the top speed end, it's well worth the $14 - but consider your driving area and habits...if you are jumping from red light to red light in the city, it's going to be much less help than longhauling it down the local main drag.
Still worth $14, tho, yes.