I think you'll be sadly disappointed if you expect a four stroke of equal power to improve you duration. I have a Saito 72 and it's quite thirsty. My most efficient engines are two strokes (Webra 120 and OS160).

Unless you get pretty tricky, there are only two basic ways to improve duration: 1) reduce power, 2) increase fuel capacity.

One thing you may consider is using a small carburetor and a large prop. The idea is to get the wot speed down to 7000 rpm or so. This could also improve basic efficiency depending on port timing in a two stroke or cam timing in a four stroke. I think one of the reasons the Atlantic team used old OS four strokes was because the cam timing was milder back then. They also used a tiny carb among other things