I strongly urge you to obtain a copy of the official aircraft record from FAA-OKC which you can order online. www.faa.gov
That record may have a copy of the Form 337 which lists the particular STC under which that engine was installed.
The veracity of the following information may be suspect:
Robert (Bob) and Barbara Williams owned several STCs of this nature under several names. Their reputation for response and fiscal responsibility is not good, according to many commenters on many aviation websites. (You can do your own search on the Internet.)
The last contact information I have is:
Bush Conversions is located at Po Box 341 Udall, KS. Phone: 620-782-3851
Avcon Conversions,PO Box 654,Udall KS 67146,316-782-3317