Ys 45 weight with muffler?

Regardless of the (small) weight difference, I would take the 4-stroke route. I have Saito 72 in mine -- Ys 63 would be still a better option.

All the top 3D fliers around the word use 4-strokes in plane category. Take a look in the web. Ys 63 is the most widely used. Why think differently?

These 4-strokes have no shortage of power and their reliability and throttle response are superior.