To get more horse power from an engine, one thing you can do is have more power strokes per minute (more rpm's). There is a limit to how may rpm's you can have and one of these limits is piston speed i.e. how fast you can make your piston travel before it decides to go "nah". A.Bell, states "You can not arbitrarily say, for example, that all single cylinder 125s can be safely run at 12,000rpm. As a starting point, run the engine initially at a speed equivalent to a mean piston speed of 3800 feet per minute and work up from there to a maximum of about 4500 feet per minute.

Wikipedia states, "The mean piston speed is the average speed of the piston in a reciprocating engine. It is a function of stroke and RPM. There is a factor of 2 in the equation to account for one stroke to occur in 1/2 of a crank revolution (or alternatively: two strokes per one crank revolution) and a '60' to convert seconds from minutes in the RPM term." http://en.wikipedia.org/wiki/Mean_piston_speed

mean piston speed (in m/s) = 2 * Stroke * rpm / 60

Below is the calculation for a Vespa PX 200 with rpm, metres per second and the equivalent feet per second, as well as the speed in km/h based on standard Vespa PX gearing. From this we see that with a 57mm stroke the Vespa engine will not be in the danger zone till 11,000 rpm. A caveat on this is that Jennings (1973) states that limit should be set around 3500 feet per minute (10,000 rpm). As a lot of Vespa tuning tech is actually from the 70's and early 80's, this is probably the correct value to stick by.

It is very hard to get to 10,000 rpm in 4th (and even 3rd), however the above is a good guide should other modification get you in that region.

Below is additional information for a long stroke (60mm) Vespa engine