http://rgl.faa.gov/Regulatory_and_Gu...t=1&Count=100#
The STC regards installation of a Sky Sign Aerial Advertising System.
In reviewing the STC, it does indeed appear that effectivity includes only those aircraft that have both an engine of 150 horsepower or greater, and that have STOL wingtips installed.
The original poster's question regards those aircraft that lack STOL wingtips. In such a case, an approval will need to be had to install STOL wingtips, or an effort may be undertaken to get DER/DAR assistance in securing an approval without the STOL wingtips as a field modification; that may be possible with the assistance of the original STC holder, at least insofar as the use of that particular STC.