I know that with things like the barrels, you dont completely repaint them. You clean up the rusty spots and then paint those spots with the closest match you can get.
Thats the preservation part.
Its never going to match, but anyone who knows what they are looking at will be able to see the original paint was left and the barrels are being well looked after. Same thing for any other painted surface. You do the best you can to save all the original paint that you can, but foremost must be the preservation of the machine as a whole from further deterioration.
I know you are still mulling over what to do with the bike, and the above is a metric-ass-ton of work.
You could still completely restore the bike using all the original parts, get all the original hardware cad plated, get the tank rechromed, etc.
Or just rebuild it, and only replace parts that fall off or fail.
Whichever way you go its a beautiful bike, and its going to be fun.
Lee