And not all nickel coating is the same. Nickel can be a strong and durable finish. But many coaters who do nickel don't do it to an industrial strength and it's not corrosion-resistant enough to endure.
There's a shop not too far from me that I investigated having a frame done. While they do offer the service (it's the same place John Ryland of Classified Moto uses for his tanks) they strongly discouraged doing it as what they offer won't hold up (nor will his tanks apparently
). I was told I would have to find a player who specialized in nickel as final coat (nickel being used in the chroming process was not the correct method).
If it were me, given how damaged that bike is, I would opt for repairing as much of the tubing as possible, then polishing it and shooting it with clear powder coat. That would give you the best appearance, the greatest durability, and longest lasting finish. Hogwash that powder coating will warp or damage your frame! Absolute rubbish that opinion....
I've had sheet metal powder coated, aluminum sheets coated, frames coated, and not a single MM of distortion. The only warpage I've ever encountered was welding stainless bungs into thin sheet metal (vintage tanks) for new filler necks. And even that was minimal.
I get you want restoration correct, but as your tank is already destroyed, your objective might be to "recover and recreate" as closely as possible a new example of the vintage bike.