Get rid of those old tires!
Put a set of Zippys on there, I have them on my Elite and my Vespa - they handle quite well and aren't expensive like 'real' motorcycle tires. You have no valid excuse
for not changing to new tires.
Look on eBay for drive belts. You can get a deal there.
+1 on the slower 125 accleration. Threw a 125 in my 150 when I thrashed the splines on the variator shaft (that comes fromt he bottom of the crank).
The stock accleration is absatively GLACIAL above 50mph. The 150 got me up to 58 mph top speed also. Mechanical limits on them going faster? I'm sure you can make them accelerate quicker, but not get a higher top end.