RE: Which 4 stroke engine do you recommend?
Go with name brand for 4-stroker, nothing's worse than fussing with a temperamental motor.
OS is tops for being user-friendly, and I find Saito has a little more torque given the same displacement. Saito may take a little longer to break in. After the break-in, both are easy to tune. YS has more power, but is a little more touchy, especially for break-in.
Parts and service should figure into the equation, and all are great there.
Good on you for going for scale sound/realism, nothing like a 4-stroker for that!
Ed