Tap gently from below, although not on the threaded bolt, place a large socket set piece over it first then tap
All bearings are standard shimano ones, for that age they will be caged loose balls probably. The lower race is held in by the silver race shim that pace used - don't loose it! The top bearing adjustable race will probably be stuck onto the stem/steerer but will come off with a gentle tap.
Nothing will be screwed in, the top bearing adjustable race has been reamed out so there are no threads in it despite the fact it still has spanner flats on it. Pace invented the aheadset a good couple of years before Diacompe did, although the Pace one was a little more homemade!