Dsoul, 60kph/37ish MPH is not bad speed out of a two stroke 50. You can get a bit better by replacing the CDI, & upping your jet size.
Sorry I cannot provide specifics, but they're here on the forum somewhere if you google it or do some searching in the forum yourself.
http://www.kymcoforum.com/index.php?topic=6198.msg62708#msg62708 is the big one as I recall.
Yeah the big bore kit would get you a chunk of power as well, but more at the low end from all tell. I had an old moped with a 70cc kit from the era (long time ago) in it that definitely had nice low end, but still topped out (unrestricted) about 45ish mph with a long ramp up. Acceleration was good up to city speeds, but really took a while after I got to 40.
By your choice of metric units I'd assume that you're not in the U.S., so I'm not dead sure of the laws there, but in the U.S. even the big bore kit would require a license. Really in most areas you can get pulled if you're going over 30mph on a "moped" license or local equivalent.
All that noted, unless you have a licensing restriction of some kind or other you can forgo all the messing around and pick up the 125 or better version of the agility of any number of other bikes and get the same thing with no loss of MPG, no messing around, & lower detonation factor then running fully topped out a on two stroke all the time.
My old S200 was great up to about 50-55mph, & topped 60ish. This is pretty consistent for 150ccish bikes; albeit it was a 163cc, & just like with the 70cc kit I had a bit better bottom end. The 125's are a bit slower, & my SYM's a bit faster.
