Using the cab of one aircraft and the components of another aircraft used to be common practice in the industry until the FAA changed their mind a while back. But really, what would the harm be? There's no life-limit or even overhaul-interval on the cab. When we look at the "age" of a helicopter, it has more to do with what configuration it's in...if all of the pertinent mods have been done. I used to fly an early-SN 206B that had been a sprayer. And what sprayer hasn't been wrecked at least once? Oddly, it had a late-model instrument panel which was not "period-correct" (as my antique car friends say), and there were a few other configuration changes that made us wonder: Was it the actual fuselage? Did somebody go to all the trouble of cutting out the old instrument panel with the individual push-to-test caution lights and tiny pressure/temperature gauges and go to the trouble to install the late model panel, caution lights and gauges? Hmm.
Again, it hardly matters, because helicopter are a collection of components with various lifetimes. The cab is just the Christmas tree we hang all the ornaments on. So if you could buy a bare fuselage assembly from Bell (I'm sure there's a PN for it), and then stick the old tail boom and a bunch of airworthy components on it, and a data plate from the helicopter all the components came off...what's the harm? You'd have the old helicopter with a new cab. But nope! Not allowed anymore.
Sounds like Harper's case may be a tad more complicated than simply swapping out the fuselage from one 206 to another. Sounds like there's more to this story than that. Sounds like the feds were onto him for a long time - even going to far as to actually buying one of his aircraft. It would be interesting to know the whole story.