And it's still missing and still down on power?

If so, the next question is: How much damage did the motor sustain when it overheated?

Check your coolant for traces of oil and vice versa. Also, keep an eye on the coolant level. If the engine is consuming coolant (i.e., it is not leaking anywhere) that is a clear sign your head gasket has blown.

If that's the case, it's possible that the cylinder head may also be warped. The head would need to be machined flat once again, or if it's really bad, the head may have to be replaced.