I have an '80 102sp an just recently had the exact same problem as you with the petcock(main /reserve valve) leaking. I took the petcock apart to find that the rubber gasket inside it had bubbled and cracked. The gasket is a circle with three holes in it, you'll know what I'm talking about if you take the petcock apart. I made my own replacement with little 1/16"-1/8" thick sheet of rubber I had laying around in my garage using a hollow punch and some tin-snip scissors. So far it hasn't leaked a single drop. Making this gasket wasn't easy, so if you're not much of a craftsman you can buy a whole new petcock at mopedwarehouse.com for $18, which isn't too bad. Good luck with that.
As far as performance from removing parts, my friend took his entire muffler and airbox off his 102sp and it was definately faster, but i wouldn't run it without an airbox all the time. Taking the muffler off makes it A LOT louder, so if you don't want to piss off the neighbors it might not be worth the small performance increase. I took off my front fender, it looks and feels better to me. I don't ride in the rain so I don't really need it. If you're looking for cheap things to help performance, after taking apart and cleaning the centrifugal clutch and then torquing it down to 30 ft./lbs. I noticed an increase in both acceleration and top speed. Make sure the belt isn't slipping and that the chains are lubed. Having the idle slightly high helps a little bit too when riding, it make the power more available if you suddenly get on the throttle. Also, check the exhaust pipe coming from the exhaust port and see if there's a wall of carbon built up around the inside. I have this problem and I'm sure it's robbing me of power. I plan on throwing the pipe into the bbq sometime soon and seeing if i can get it hot enough to burn most of the carbon off.
After having the bike for almost a year, I've come to the conclusion that if this bike is stock it won't be fast. It'll never get to over 30mph on flat ground. That's why I'm planning on ordering a 70cc kit, carb, exhaust, and maybe some sprockets from 50cc.nl to up the power and acceleration. It still won't be "fast" but at least I'll be able to keep up with cars driving around my neighborhood and not have to worry about the engine dying while going up hills.