In regard to the headset, probably the best course of action if you want to replace it is to have a shop do the work.
If you are inclined to do the work, you'll need at the least a digital caliper.
1) Remove the headset cups. Measure the diameter of the headset cups. Measure the interior of the head tube in at least two spots and average the readings.
2) Remove the crown race from the fork. Measure the diameter of the crown race in two spots and average.
3) Measure the length of the head tube.
4) Measure the length of the steerer tube from the race seat to the top.
5) ST length minus HT length gives you stack height.
Stack height, crown race diameter and cup diameter determine what headset you'll need.
According to Park Tool, you'll need a difference of 0.1mm to 0.25mm between the cup diameter and head tube diameter for a proper press fit.
My guess? Tange Passage.