The only "Official" equivalent was the AMA standard of several years ago. By that, a 1.0 four stroke was considered the equivalent of a 0.6 two stroke. That same ratio was applied to all classes. For example a 50 four stroke was legal in class "B" where a two stroke had a 0.30 max size.

With modern four strokes the average two stroke still has a slight edge over the same size four stroke, but it's pretty much up to you now. An 80 two stroke or a 90 Four.

Bill.