The recommended engine size for a Keil Kraft Super 60 was 2.5-5cc according to the advert on the back of my July 1964 edition of the Aeromodeller, so a 15-29 two stroke.

If you were planning to fit a four-stroke, I would be inclined to try an OS 40 but the new 30 sized SC four-strokes should fit the bill.

I intend to build one of these over the winter.