Myself, I'd complete a very careful assessment of this hull before committing to it.
The Hunter 140 hull construction is a plastic skin around a foam core, not a solid or cored fiberglass hull, so first you should assess the hull very carefully, because this sort of construction is much more likely to be damaged by heat than a FG hull. It's also near impossible to repair this material. Check for deforming, soft spots, blistering, cracks.
Assuming the hull is intact, the next item I'd check is the cost of putting a new (or used) rig on it. I imagine that the cost of an all-new-rig complete with mast, boom, stays, sails, hardware etc would be quite prohibitive.
All told, you might find that it's alot cheaper to simply buy a complete used 14' or 16' sailing dinghy than to rebuild the 140.
Past that point... for a mast, I would be inclined to just buy a used aluminum mast & boom from a good boatyard. Most boatyards or dealers seem to acquire piles of this stuff. Every so often they even give the stuff away.
It would be fairly simple to make a serviceable rudder and centerboard out of marine plywood. After assembling and shaping the part, put on a couple coats of glass cloth and epoxy, some white marine paint and you're off... I'm sure you could get the dimensions from Hunter or a dealer.
Good luck !