I hadn't heard that Saito had discontinued the .65. Have they got a replacement? There's nothing between the .56 and .72, that leaves a gap in a significant range. If they are coming out with a .62, it should be fine. I'd go with the .72, myself. It is lighter than the .65, makes more power (not that you'll need it), and is the same size. If you use the bigger engine, you can more easily swing a scale diameter prop (14-6 is what I use on my 1/5 Sig Cub, and I have the Saito .65 in mine.(inverted, fits completely within the cowl)

I just checked the specs on Horizon's site for the new .62. All it is is a bored out .56; the stroke is the same as the .56 (19mm), but the bore went from 24.8 to 26.2mm. It ought to be a real revver, but I don't think it will be as torquey as the .65 was. Either the .62 or the .72 should be okay, I'd lean toward the .72.