I would agree with the Saito .82 or even the Saito .91. You can't compare props, since the two stroke needs a smaller prop to turn up and get on the pipe. I ran an OS .50 SX with a tower muffler on my UCD and like the performance better than the four stroke with the larger prop, more P-factor. But to each his own. IMHO it takes at least a .80 - .91 four stroke to match the output of a strong .50 two stroke.