Let's see... The 2005 Paris Dakar saw the KTM 660 really mop up. At 131 kg, it's maybe a little heavier than I'm looking for. And it's not available in the US.

Several 450's placed okay though, and the KTM 450 SMR IS available currently, weighs approx 240lbs ready-to-race-sans-fuel, and comes without an electric start! MSRP $7500. Here's a picture of it nekid.

Wait wait wait wait! The 525 SMR is only one KG more!