CKD has little chance in meeting "Buy American" standards - they have never been awarded a light rail contract in the US and do not have any manufacturing capability here.
Kinki Sharyo, Bombardier, or Siemens seem far more likely. All have experience in the US market, all have the capability to manufacture or at least perform final assembly in the US, and as far as Boston goes, all have (or will soon have) rail vehicles operating here. Kawasaki is another possibility.
Idle speculation here, but what I would do is tap KS for another "emergency" order of 25 or 30 Type 7s, if they can be delivered in a year or so. That would allow for the retirement of the Boeings plus fill in for the Type 8s we're stuck with to get worked over a few at a time until they start performing acceptably. Meanwhile, get the ball rolling on a new order of 50 or so low-floor cars from someone (maybe Kinki, maybe not) to make up for the loss of the remaining Breda order.