RE: Bates 106
From one of your pics it doesn't look like the g62 would fit. If you were able to shoehorn it in it does not look like it would leave much room for a fuel tank before you hit the wheel. The da50 would swing a prop size close to the g62 and a dle30 swings a 18" prop like a g38. You would probably save a little weight with these lighter engines and put some lead way up front of the fuse where it would make the most difference.
Stretching the wing is not a bad idea but I wouldn't add it at the tips it looks like it has a steep cord taper and might make the tip too pointy. Adding to the root or stretching like you thought might be the best bet.
Mike