I've got one of the OS FS .72a 4-strokes on my Great Planes Big Stik 40 which weighs in at 5.57lb and with a 12x7 prop spinning at approx 10,800 it really hauls it around.
I was thinking of trying the same engine in an Osmose 70 but i am also thinking that the OS FS .81a with pump might be a better choice with the inverted install and tank on the CofG.
I have the OS 81a (non pump) in my Osmose and I love it. 72a will probably be a better choice to save a bit of weight. Use the 90 degree header and you can can keep everything nicely in cowl. Downside of course is OS is expensive. Each to their own.