I think that these a/c are basically classed as either ultralights or advanced ultralights. As for aerobatics they'd be extremely limited or even prohibited outright, but I could be mistaken. I remember seeing some photo's a couple years ago of a chap who'd clad his Loehle 5151 in a light, thin plywood veneer and fiberglass and it looked rather neat. Couldn't tell you where I saw it or what it's status is.
Titan Aircraft
http://www.titanaircraft.com offers an "almost ultralight" P-51 replica, but the all-aluminium Stewart S-51 seems to be in legal and production limbo (pretty ship, though), and of course the late, lamented Thunder Mustang is pretty well kaput 'cept for a couple of racing examples.
The FEW Mustang seems to have been resurrected by Legendary Aircraft
http://www.legendaryaircraft.com and I hope it thrives; it'd be nice to see a P-51 kitplane succeed.
N.