Great summary Joe. I'm guessing there might be different requirements in different countries, but, as Joe stated, some, if not most of these, will be geared to the clearance or restrictions on flight. Some aircraft have a fatigue meter and 'G' meter fitted, where there never was one before, an example being the BBMF's Lancaster, which is also equipped with TACAN, the latter more at the behest of the RAF than anything else.
In the UK, some 'warbirds' might have, for example, a more modern altimeter or compass but, as far as I know, this is more a case of whether the original passes or fails such things as pressure tests, deviation etc. Virtually all of these aircraft, in the UK, will have a modern radio/nav set up, but owners/operators try to keep these as discreet as possible, so as not to 'spoil' the otherwise authentic layout of the cockpit. In recent years, more and more operators have made great efforts to present the intenal arrangements as close as possible to the original, not just the external appearance, and this has sometimes included 'camouflaging' modern items 'inside' original fitments. Other equipment fittings , where they are not obvious, are, wherever possible, updated, such as the material hoses and wiring is made from, fittings and unions, brake linings and so on. On the other hand, some items tend to be rather rare and expensive, as they have to be produced specially, or as a limited run if a major manufacturer, one such being landing gear tyres, an item not normally available at 'Tyres r Us'!!