I'd say if you're not looking to do a restoration, then you don't really need specific parts.
Odds are the buttons are sanwa or seimitsu (I believe the joysticks were originally Seimitsu LS-32). You'd probably do best just to get candy cab buttons, ones designed for metal are a lot shorter top to bottom.
You can find those buttons and joysticks anywhere, they're pretty much standard for control fetishists.
Switching from 6 buttons to the NG 4 is a might tricky if you don't want to just drill a hole and use button plugs. The 6 buttons are probably in a japanese arc which is more drastic than how the 4 button layout usually is.
But if you can live with that, you'd probably be looking at removing any CP artwork, remove all the current buttons and hardware, and drilling a 30mm hole (assuming you go with a Japanese button, 1 1/8 if you go with US buttons) in a comfortable place.
Next.. Sand any paint off, go down to bare metal (optional but recommended).
Next you'd tape a sturdy piece of cardboard behind the buttons you want gone (the remaining 3), fill them up with Bondo, try to avoid air bubbles. Let it cure over night to 48 hours per directions. You could also do this reverse and use fiberglass on the inside for more strength.
At any rate, sand down the top of the bondo so it's flush. 2 coats of good metal primer, 2 coats paint of your choice (not too much because the holes will shrink). Re-install hardware and rock.
Now if you're NOT okay with the current arc of the button layout then... Yeah, I don't know if bondo will hold if you fill in the 6 and re-drill all 4. In my head I see that being no fun.
You could just drill the 4 holes as best you can and hope a plexiglass overlay is strong enough to support everything. I've seen cabs like that, where they just drill 4 new holes and leave it up to the cp overlay for strucutal support of the buttons.
