I'm pretty sure what the difference is that.... I have no idea lol. He sells both the 49 and the 60-ish cc motors, he also uses the same engine as the basis of his 'Lil Evil' kit, which is an F(s)80 with some soup poured on it.
I've bought a few things off the website, but usually I just call, I'm a red neck, I like to flap my jaw with folks, especially if they know stuff I don't. Just give him a ringy dingy, he's answered the phone for me at 7:00pm on a Saturday. If he's there, he'll answer, he was working on a customer's gearbox. THAT'S customer service!
If we all piss and whine, maybe he'll finally fix the site...lol
If I'm not mistaken the difference between F and FS is that the F denotes his "block" (which is actually a running engine without carb and exhaust.) whereas FS denotes his kit. I could be wrong. In all honesty, the key to figuring out his site is to use it like a photo inventory of his stock, then call him. That's how I do it, picture, price, phone. Primitive, I know, but they don't call me Kaveman for nothing.