You typically add ~50% to the engine displacement when going from 2 cycle to 4 cycle nitro engines. That's just a "rule of thumb", as mentioned above, different engines of the same size can perform quite differently. Some examples of the 50% scale up: a 40 sized planes take .46 sized 2-strokes, and .70 sized 4 -strokes. 60 sized planes take .60 2-strokes and .90 4-strokes. So a 1.8 4-stroke could be scaled down to about 1.2 2-stroke. Your Moki 1.34 should be fine. I've got a Twist 150 that specs a 1.25-1.8 4-stroke. My Super Tigre 1.4 flies it with plenty of power to spare, and I've heard that even an OS 1.2 has plenty of power for the Twist 150.