AT 70K miles you should not be having mechanical problems on a normally aspirated car. I would consider other things other than valve or ring issues. From your description everything was fine until you installed the header. You also described a change to the EGR tube.
Loosing a quart of oil in a short amount of time is not normal for a Miata either.
You also said you had changed the oil about 500 miles ago. (have to ask)... Is it the correct filter?
There are ways to check for head gasket problems. I can think of a few but I will let the real mechanics speak up on the procedures best for Miata's.
I will be optimistic and say the motor is still good.