I am rebuilding my big trainer to use a SAito 130 twin which I recently acquired. I have only the rear part of the plane which is just as well. It is actually a Skyward 120 arf , essentially their 60 blown up 150%. The original was 20 lbs with a OS 120 . It definitely flew on the wing, but it was an awesome plane to fly.
The original foil has a little less than 2% camber and is semi-sym.
It is about 13% thick. I feel that this foil gives up too much lift, I flew the 60 size and felt the same thing.
I ahve a big Edge so this will be my floater, might even get floats for it.
So does anyone have experience with some "flat bottom foils" like the NACA 4415 or true Clark Y. I built a small Cub with the S7055 but its only 10.5 % thick.I am upgrading my Compufoil to 2000 so I can hardly wait. This is going to be a fun project after doing the Edge all winter.
I might add my target weight will be 13-14 lbs, & I'm usually close.

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...

Your right about the flaps Matthews, I will definitely be setting up the wing with them. I think I might just go with the Clark Y to get the project rolling even though its not on C 2000 list but I will go dig it up.

If you need a clarky.cor file I can email it to you.