Of course it is.

You can practically get all the parts off the shelf today.

What you can't do is fly one without a pilot's license. Or manufacture anything larger than the ultralight class without government certification.

And even in The ultralight class is extremely difficult because they're really only good for a payload of one person if they're not a fat ass. With a very short range to boot.

There's actually numerous open sourced ultralight class helicopter plans floating around the net. That can be built with very few machined parts. But the second you try to add a bigger engine or larger fuel tanks You're out of that range that the king allows.