I would imagine that at 20 lbs the airfoil just might "have given up too much lift". But if you can shave off 7 lbs then the original should be just fine.

But if you want to go for it the proper ClarkY has some very good numbers attached to it. On the other hand if you don't want it to accelerate too much when you put the nose down a thicker option like the 4415 would add some drag to keep the speed more constant.

And your rebuild sounds like a model in need of proper flaps. Get rid of the strip ailerons, if that's what it used, and add a sub spar at the 80% point and use the outer 40 % for ailerons and the inner 60% for flaps. Adds lift AND WASHOUT when needed for lift and stability and tucks up when you want to GO. And the full 90 degrees will make for some interesting short field landings.

PS: I'm surprised no one addressed this post earlier. Guess they were all waiting for me...