A .56 will be just fine, plenty of power. My H-9, I believe, had a wooden engine mount, a couple of rails that fit in square holes in the firewall, and some bracing. Mine is the older ARF, yours may be different. I'd recommend filling the holes with epoxy, and while you're at it, reinforce the firewall attachment to the fuse, and then get a nylon or aluminum mount for the Saito. Dave Brown makes a nice engine mount, it has a round flange and you can mount it any direction. Just make sure you ask for the 4-stroke mount, the rails are longer than the 2-stroke mount.

If you go with the different mount, just check placement and keep it the same as the factory location. Great flying plane; I have a .72 in mine, with floats it weighs 11 pounds. I've flown 25 minutes on a tank (the stock one) because I rarely go past half throttle.