HI, my 2 cents... yes, that thing looks like having parts from 2 or 3 different headsets.

Take the headset out completely, and as you said measure the internal diameter of the head tube.. Hope you have a nice METRIC CALIPER, u can do it in inches but it will be harder to guess the metric size, in the unit conversion some decimals will be lost and u might end getting a headset that might fit lose or wont fit in the head tube.

Sheldon brown has a sticky with headsets. I dont know how old that bike is, and if it is too old u even have the chance to have nice french sized headtube and fork (my opinion). Best case scennario, u are looking for 30 mm or 30.2 mm internal head tube diameter. It could be less too.

Get the metric caliper 1st.

An steel headset can be like 20 bucks maybe.

Good luck