It isn't anything to do with LAMS what you can and cannot do, it's the ANO with more explanatory info in CAP520 Light Aircraft Maintenance as Mixture says (see below). I'm not sure that Euge actually wants to do the job himself though, more just wants some work done.
I presume that you want to have the seats recovered rather than actually purchase complete new seats. There are a number of specialist firms which will do the job for you (and recover the sidewalls and fit new carpets). Most will tend to the 'springing' at the same time and repaint the frames. Make sure they are thouroughly inspected for cracks.
These days in EASA land if the seat covering doesn't have a Piper part number on it then it is a mod and drawings have to made for approval etc. Whether many PA28 owners actually go to this extreme and pretty much double the cost of the job, I couldn't possibly comment....
PA28-161 Warriors have a rear bench seat and that is your lot, I'm afraid. Archers and Arrows (and PA28-140s) have individual rear seats but you can't change one for the other. I doubt anyone has done a mod to change the design, just not worth it.
Lap strap seat belts aren't too expensive to buy new (Piper Parts UK are at Shoreham) and are normally made by Am-Safe. The inertia reel shoulder harness are made by Pacific Scientific and their European place is at Maidenhead, Berks. They can overhaul them totally legally for about £100.
Sun visors are also available from Piper (as others have said, there may be mandatory placards on them which must be displayed somewhere in accordance with the Flight Manual). There are after-market sun visors available which if they are an EASA approved STC (mod) then can be fitted.
I strongly recommend that you discuss your wants list with your engineers and let them do the work. You can cause an awful lot of problems with aircraft if you don't know what you are doing. Sun visor, seat and seat belt removal are not long jobs but things like the seats need to be refitted properly....
Air Navigation Order
Section 3, Part 4.12
Pilots maintenance—prescribed repairs or replacements
12. For the purposes of article 16(7)(a), the following repairs or replacements are prescribed—
(a) replacement of landing gear tyres, landing skids or skid shoes;
(b) replacement of elastic shock absorber cord units on landing gear where special tools are
not required;
(c) replacement of defective safety wiring or split pins excluding those in
engine, transmission, flight control and rotor systems;
(d) patch-repairs to fabric not requiring rib stitching or the removal of structural parts or
control surfaces, if the repairs do not cover up structural damage and do not include
repairs to rotor blades;
(e) repairs to upholstery and decorative furnishing of the cabin or cockpit interior when
repair does not require dismantling of any structure or operating system or interfere with
an operating system or affect the structure of the aircraft;
(f) repairs, not requiring welding, to fairings, non-structural cover plates and cowlings;
(g) replacement of side windows where that work does not interfere with the structure or with
any operating system;
(h) replacement of safety belts or safety harnesses;
(i) replacement of seats or seat parts not involving dismantling of any structure or of any
operating system;
(j) replacement of bulbs, reflectors, glasses, lenses or lights;
(k) replacement of any cowling not requiring removal of the propeller, rotors or
disconnection of engine or flight controls;
(l) replacement of unserviceable sparking plugs;
(m) replacement of batteries;
(n) replacement of wings and tail surfaces and controls, the attachments of which are
designed to provide for assembly immediately before each flight and dismantling after
each flight;
(o) replacement of main rotor blades that are designed for removal where special tools are
not re(p) replacement of generator and fan belts designed for removal where special tools are not required;
(q) replacement of VHF communication equipment, being equipment which is not combined
with navigation equipment
I've tried for years to offend A&C and not succeeded yet....