some of the really old homelites you can't take the square adapter off. If it's not an antique though you should be able to take it off. Check out the site in my signature to see what I did after that.
PS. Welcome to the forum.
I would think some keystock with the other end potted into a shaft with silversolder and stabilized with a bearing plate mounted to the motor would work for you.. Maybe you could cheap off and pot it in JB Weld and put a couple holes in the shaft to keep it from spinning.