My Chipmunk is a bit over 8lbs. it's been crashed and rebuilt a few times. I have a Saito .91 on it and for me, it's a bit under powered. I'd think that the ST .90 ( I have one of those on my Hobbico Sukhoi SU-31) would pull it around nicely!

I'd probably be conservative on the lightening holes. Don't want to weaken the structure of the airframe. Beyond that, I'd say you're headed in the right direction. I love the way this plane flies. I'm quite likely to buy another one if this one has another gravity incident. ;-)