+1 on the Powder Coat. Probably the only way to be sure....
That nickle plate would be sweet, but probably still require a clear of some sort, right?
U-Pol system 20 2-part clear is what I use, and works great on paint sanded to 600. Not sure how it would work on bare metal though. But for $40 a quart including catalyst, you can't really beat it.
At my shop, we coat raw steel (with the scale still on from the yard) with a product called Penetrol from time to time as a finish. Just wipe it on like a wood stain and let dry. Cheap and easy rust preventative that lasts a long time indoors. Not sure about on a motorbike, but worth a shot. It might darken or yellow a bright-sanded tank, but so would WD-40.
Anyway, there is my tiny bit of knowledge. Good luck! And post some results!