To have an STC approved on your very nice aircraft is going to cost you an arm and a leg. Petersens spent 150 Hrs per engine to have the various models cleared for operation using Mogas.
A few questions - I assume you're in one of the Benelux countries. Are the fuels there free of Ethanol? I really hope so for your (and others) benefit.
What is the AKI (Anti Knock Index) of the highest rated Mogas in your country - is 95 Octane (RON) the highest Octane there??
Which engine is in your aircraft - is it a 160HP variant of the O320? If so - I believe you just may be wasting your time if the MON (Motor Octane Number) is 87 or lower. Additionally - I assume it has a Bendix fuel injection system. Yes?? Petersens had problems getting the Bendix fuel injection systems approved by the FAA.
You could fast track your quest to find out more by contacting Rene Wenger in Switzerland: autofuel at popnet . ch (please fix up the email address).
He may know who has an STC for Mogas for your aroplane.
Hope this helps.