I disagree with FHH. A good rule of thumb is that a 4-stroke engine needs to have 1.5X the displacement of a 2-stroke for the same power. Go to the product guide here in RCU and compare the specs of a Saito .72 4-stroke to a O.S.Max .61 FX and you'll see that the .72 puts out 1.0 HP@10,200rpm and the .61 puts out 1.9HP@16,000rpm. Now, the 4-stroke will swing a bigger prop so it has a better grab on the airstream, but a .72 4-stroke is not a good substitute for a .60 size 2-stroke. The .72 4-stroke is an excellent alternative for a ~.45-.50 2-stroke. Again, this is a general rule since some engines (YS) are powerhouses for their size.