Two Stroke Racelab

  Two-Stroke Performance Tuning Book by A. Graham Bell (Haynes Publishing, 1999)

  Tuning Vespas, The Norrie Kerr Way (Scooter and Scooterist, 1996). 

  Motorcycle Tuning Two-Stroke by John Robinson (Newnes, England, 1994)

  Two Stroke Exhaust Systems by Roy Bacon (Lodgemark Press, England, late '60s).

  The High Performance Two Stroke Engine by Dr. John C. Dixon (Haynes Publishing, 2005)

  Improving Two Stroke Engine Performance by J. W. Vierdag (Longemark Press Limited, Kent, England)

Tuning your Two Stroke Engine by K. G. Draper (Coles Publishing Company Limited, 1977).

1) Lambretta Tuning Manual by Dave Webster.  2) Lambretta Performance Tuning & Conversions Manual by "Scooter and Scooterist Production". A lot of info including tuning, conversions, carburettor and gearbox charts etc. from the Scooter and Scooterist Production and the well known Dave Webster.