If I'm envisioning your wing correctly, you'd basically have to get new spars and a few new ribs, and rebuild to original configuration. I would definitely take it back out to stock span. Eight feet is a LOT of lost wing area. I don't see a clipped 18 doing much of anything very well.
Did they change the strut attachment point? If so I would follow the above advice. If they have the stock strut attachment you should be able to add spar length as long as the splice is beyond the stock strut attachment point. Lots of bent spars laying around you could get 4-6 feet out of.
DENNY
Which end of the wing was clipped, inboard or outboard? If it was the outboard, just splice on a new section of spar using Piper's approved spar splice method. If the inboard end, there will be a lot of work involved including changing the angle of the strut fitting on the fuselage along with new struts.
Farmboy has a clipped wing Wag -18 which matches your description.