KDX Top End
The Kawasaki KDX 200 or 220 looks like a realy good fit for a Vespa topend conversion due to stroke and bore being almost the same. However, there is quite a bit of modification to do to the cases due to the height that the cylinder sits on the PX cases. The 220 is a lot harder to come by so we chose the 200. If we have to go bigger we may be able to get a 220 sleeve and do some porting. Another alternative is oversizing.
Scooterhelp talk about it for Lambretta's (Kawasaki KDX200 Cylinder)
The KDX also has a bigbore kit see: 240 Big-Bore Kit for the KDX200
KDX dyno (note this may be an old KDX)
Wiseco Information
KDX 200
With the KDX crank the 200 is 198.51cc, with the Vespa 60mm long stroke we end up with 205.35cc.
The biggest we can go in the 200 is 68mm bore = 217.99cc (with 60mm Vespa long stroke)
For the 220, bore 69-70mm = 224.45cc-231cc (with 60mm Vespa long stroke)
Top End Bearing
Both the wrist pin of a Vespa PX and KDX are 16mm, so a KDX piston can be used on a Vespa conrod (with a Vespa topend bearing).