I can't imagine that resetting the odometer on a replacement speedometer to reflect the previous speedometer's mileage would be illegal. Here in CA, when you sell a car less than 10 years old, you have to sign a statement that the mileage is correct, or a statement that it is not. Older than that, no one cares.
In any case, "mileage" is a meaningless concept on a car this old. "Mileage" of which part, exactly? MY 52 MGTD had at least seven documented engine replacements or rebuilds, at least one transmission rebuild, and countless replacements of other parts. Most cars have had one or two speedometer replacements or has been run with a broken speedo cable, and there is no record of how many times the most recent odometer has turned over.