My absolute favorite way to scale up a plane is to do a 200% scale printout of the plans, then double the foam on the body. The main issue of scaling up planes is that the thickness of the foamboard does no scale up with the plans. With the pietenpol however there are a few more points of stress on the airframe (mainly how the wing is attached to the body). To go about attaching the wing to the fuse I would find some wooden dowels at a craft store that are roughly 200% the scale of popsicle sticks, or get ones that are close and make customizations while building.
edit: I just remembered that the wing is also only a single piece thick, you might want to think about ways to reinforce that by either adding ribs or a metal rod (about 2-3mm diameters) bent at the same angle of dihedral of the wing and run along the crease of the foam.