Have you looked here?
http://www.sikorskyarchives.com/S-46%20 ... S-300A.php"Power was supplied by a 75 hp (at 3200 rpm) Lycoming O-145-C3 air-cooled four cylinder horizontally opposed engine. The engine was installed with the propeller shaft pointed aft and connected to the main gear box by a series of six V-belts. Tension on the belts was provided by engaging an idler pulley. In July, 1940 the engine was replaced with a 90 hp (at 2,500 rpm) Franklin 4AC-199-E air cooled four cylinder horizontally opposed engine to provide additional power. In April, 1941 an upgraded 4AC-199 engine with 90 hp at 2680 rpm and 100 hp at 3050 rpm was installed."