Complex sentence: Prior to GM's purchase of Euclid, the engine preferred by Euclid was Cummins diesels, with GM's 2-stroke Detroit Diesel offered as an option.
Split into simpler sentences: Prior to GM's purchase of Euclid, the preferred engine of choice, by Euclid, was Cummins diesels. However, GM's 2-stroke Detroit Diesel was offered as an option.