I had a early 90's Sig Senior kit with a wood slab cowl with filled-in pieces of wood top/bottom. I modified it by adding ailerons, changed to a tail dragger, changed the wingspan from 78 inches to 81-1/2 inches, added extra front half ribs on leading edge of wings (between each full rib) for more strength, reduced dihedral to 2-1/2 inches, installed wing hold-down boltspush/pull system on enlarged rudder and installed a .82 4-cycle engine. Much of the listed modifications you can not do wit a ARF.