To answer your question it would help to know what you expect to gain by the modifications. About the only thing the change to a NACA 1412 airfoil will do is allow the airplane to fly inverted with a little less nose up attitude than with the flat bottom one. It won’t have much effect on other flight characteristics. If flying upside down is of great importance to you, by all means make the change. Otherwise just go with the wing as designed.

Over powering by installing a G-26 is a little more involved. If you just want to have unlimited vertical climb the G-26 is probably overkill. If you are looking for good 3D performance, the Citabria is not a particularly good choice. The scale tail surfaces are too small and the wing loading will be too high, especially with the increased weight of the big engine. On the other hand if you want pattern (or more scale) type aerobatics It would likely be better to stick with the .60 (or maybe a .78) size glow engine and keep the weight down as much as possible. For aerobatic performance, beyond a certain point, keeping the wing loading down is more effective than adding power.

If you do go with the larger engine, I would start out with the same thrust setup as called for in the kit. If the cg is in the same location, downthrust isn’t likely to be required. A larger prop may require a little right thrust but only flight test will tell for sure. I would build the firewall straight and provide some adjustment that can be made during test flights. (Washers behind the engine mount are effective.)