If you wish to convert your Petter AA1 engine to truly variable speed (from tickover to full speed) you will need a fair few parts to achieve this, therfore you might like to try:
jimp@perkins.fsnet.co.uk
He specializes in post WWII Petter, but is also likely to know where to
find anything prior to that.
Jim is a retired Petter service engineer, and has loads of parts for the Petter AA1 engine.