IIRC the structure for the rear seats is a bench seat so I would doubt that you could convert this to two separate seats.
The sun visors and inertia reel seatbelts could probably be picked up from Adams Aviation supplies or any other reputable parts supplier - Google Aircraft Parts Suppliers for an endless list.
Best option, consult with the chaps you're going to get to maintain the aeroplane, they will have lists of suppliers and will be in the best position to advise on what you could get away with.
HTH