If you have any 1/8 balsa sheet, just cut out a piece the size of the top deck between the front of the cockpit and the firewall. Make sure the grain runs front to back, and just glue it on top of the current damaged piece, then sand the edges to blend it into the sides. The 1/8 extra thickness should not matter. You could also cut out the entire piece that is broken (Stabbed ) and make a removable panel to make tank insertion / removal easier. You are building the plane, so I am assumiong you have the skills to do the mods.

Don't beat yourself up on this, we've all had episodes where our gray matter has temporarily vacated the premises. Just to be safe, use a dull screwdriver next time

After re-reading your post, sounds like the access panel is the way to go, this will give you access to the blind nut. Just measure the length of the tank, as make the panel about an inch longer. Just don't go past the fuse former. Add some hardwood strips to the sides of the fuse(inside) to screw the panel on, and use servo screws, or something similar with a small washer. When you re-mount the engine mount, take your dremel tool with a cut-off wheel and trim the ends of the screws that exit through the blind nuts so they are flush.

The 4-Star is a great plane to modify and kit-bash, it lends itself well to repairs, too.