It is a misconception that 4-strokes are heavy. A Saito, OS, Magnum .91 4-stroke is lighter than any of the popular .61/.75/.91 2-strokes engines. The Thunder Tiger .91FS weighs the same as the 2-strokes.

Your OS .91FX/Magnum .91XLS/OS .61FX and most of the other .61-.91 size 2-stroke engines weigh 25-26 ounces. I have , or have had, OS .91FX, Magnum .91XLS & OS .61SF and they all were about 25.5 ounces.

The Magnum .91FS, an OS .91 clone, weighs 22.5 oz.
The Saito .91 weighs 19.7 oz.

These are with mufflers, prop washer & nut, everything that came with the engine that you need on it to fly. The weights were taken on a digital scale.