I have no Mogas STC.
The lack of an STC is an indication that either your aircraft type has been found by someone to
not be able to accept Mogas for some engine or airframe reason, and thus not approved. Or very simply, it is an aircraft type which has not
been shown to comply with the applicable
certification standards with Mogas.
That would mean that if you want to operate it on Mogas, you have to do the work to demonstrate design compliance, or demonstrate it is safe and airworthy, as the case may be.
For non certified aircraft, a good clue would be: Is there an STC available for that model of engine? If not, the effort of Mogas trials would be an uphill challange.