From the HH site (emphasis added):
The P-51 Mustang 60 can be powered for performance using a variety of 2- or 4-stroke glow engines or a small 20cc gasser like the Saito FG-20; it is perfectly powered by an Evolution® .61NX engine.
I'm still waiting to see one of these come in under 11lbs....
My battery packs (2x 4S4000) weigh about the same as the FG-20 (~30oz), and I've got a Power 110 motor that weighs 17.5oz, plus a couple of oz. of ESC up front. I still had to get the batteries within 1/2" of the firewall to get it to balance, so I figure you would have to add nearly a pound to an FG-20 setup to balance it. No way that's coming in under 11lbs.
Mine weighs 11.5, and remember I've got no fuel, and no RX pack to add to the numbers above.
BTW, it flies fine at 11.5 (and I'm at 5000ft), so don't worry too much about keeping it under 11lbs. It's just ridiculous that they advertise the plane as 9.5-11lbs.