Kostas1...

The OS 4 stroke will be a good choice. I think the Saitos make more power and are a bit lighter. With a Saito 82 balance is no problem. With the heavier engines you will probably have to move the battery in the back half of the fuse. Not hard, but a bit of work.

You'll be much more satisfied with a 4 stroke engine in this plane. Generally the 4 strokes are even a bit lighter, due mostly to the muffler.

The 4 strokes are not hard to use at all. You'll be pleasantly surprised how good they run. Especially inverted. Sometimes the 2 strokes have a problem inverted. The UCD requires the engine to be inverted.

Thanks
Barry