Are the flaps/ailerons the size shown on the plans or are they wider? If they are wider then cut them down to be the right width. If they are right then you need to build out the wing tip some. If the wing tip is not glued on yet then cut the trailing edge and shove a piece of trailing edge stock in it. Push the wood through the cut you made until the wing tip is wide enough. Then trim and sand everything to blend and glue the wingtip on.