Good comments from High Plains. I recommended staying with the original root section because it will fit the wing saddle on the fuselage w/o modifications. I don't remember any bad flying characteristics from the under cambered tip racers other than the later planes with real airfoils and less washout were quite a bit faster and turned much better. These early planes were all flown with a fairly forward CG where later designs tolerated a much more aft CG and flew like they were on rails.
Spruce, that airfoil should work fine at the tip and I wouldn't go thicker if it was mine.
I woke up a few memory cells and remembered cutting new cores for several QM 15 planes back in the same time period. Like F-1, they also had the undercambered tips on quite a few designs including the Stafford P-51 and these were changed to symmetrical sections. If I had to guess, in those days it was probably what we called a "zip" airfoil meaning we probably drew the airfoil with a french curve and if it looked right we used it. That was pretty common at the time. I've even heard of people accusing others of drawing their airfoil around their show sole. If we didn't use a zip section, we copied something that was known to work well. Things weren't always very scientific back then.
Though people like to thumb their noses at wings with under camber or flat bottom tips, I remember one extremely competitive QM-15 design that had a flat bottom airfoil at the tip with minimal washout. The design was still popular and setting records right up until the 15 class went away. It was fast and turned extremely well.