Below are some great tuning sites that deal specifically with Vespa /two stroke motors:

Worb5 from Germany - crazy modified engines...a complete nutter!

Taffspeed UK - Very reliable, reputable dealer.

How stuff works.com - Type in "two stroke" to understand the running of a two stroke engine.

MB Developments - Mainly Lambretta but they have some great Vespa pages too.

Malossi - Makers of kits for many Vespa models.

Polini - Makers of kits for many Vespa models (search under Piaggio).

SIP Scooter Shop - Great specialized and exlusive parts for tuning.

MacDizzy.com - Not Scooter related but shows everything you want to know about two stroke engines and tuning.