Good point!

For simplicity you can always assume that you are "loosing" about 30% of power on 4 stroker. Thus, to find an equivalent for the "stated" 2 stroker just multiply the size number of the 2 stroke engine by 1.3 - that's your "about the same" size of 4 stroke engine.

For example, your kit calls for .40 size 2 stroke engine, it means you can go wit about .53 (.40*1.3) size 4-stroker.

...just my penny here.