The Saito 72 or OS 70 4 strokes will be fine. You will be at the upper end of the power requirements. You will find that the 4 stroke engines of that size MAY be a little over powered for the plane (depending on the weight). You will be running a larger prop, so make sure you have adequate ground clearance (you should be ok on that). You will most likely be running a 13x6 prop on those engines vs an 11x6 you are presently using.

My Saito 72 provides power ABOUT equal to a good running/hot 53 to an average/semi tired 60.

You fuel consumption will be a lot less - figure an additional 25%+ flying time compared to the 46.

Don't forget to rebalance the plane

Good luck. I think you will enjoy the sound of the 4 stroke much more than the 2 stroke. Remember, if the engine is not putting out smoke, you are most likely running it too lean.

Hope this helps.