For crying out loud, if you are considering a .91 sized four stroke, then go with the magnum. Just as much, or more, power as the OS, and for a lot less $$. Regular price for the magnums is 199 dollars at hobbypeople.net, which saves you plenty of money to buy the pitts for it, and your still money ahead of the OS. Sometimes the magnums go on sale. I've seen them several times for 179 dollars, and we (as a family) loaded up on them when they went on sale for 149 dollars once. Never had a lick of trouble out of any of them, and they are easy to tune.

BTW, the .91 sized four stroke will haul this plane around nicely. I would recommend a .91 sized four stroke for any plane up to about 8 lbs. Over that, and you probably need to go bigger. 7.5 is really optimal, IMO.