Electric start was added to many Vespa models in the 80's mainly by retrofitting the existing engine casings to accept a starter motor which connects to a modified flywheel. Unfortunately if you want to add this to a non-electric start engine the easiest thing to do is simply find an engine with the electric start feature installed....but you can retrofit some engine casings.

Above are shots of the two starter motor types. The one labelled 'PK125" actually fits all small frame motors including the PK Automatica models. The lower one fits all PX bikes 125, 150, and 200cc engines. See the list to the left for model options.