I know there are a few MX5 nuts on here so thought it worth asking....

As I understand it these alternators are partially externally regulated by the ECU. Is it possible to retain this type of alternator when running a standalone ECU? Or does it have to be swapped for a mk1 self regulating alternator - and if so is this a straight swap?

Most after market Ecu's should be able to control an alternator. I would guess that it still has a regulator it's just the ecu is in control of the 12v exciter feed so it can kick the idle advance up a little when it kicks the alt in so it doesn't stall. Doing it that way puts less load on the engine when charging isn't needed.

Yes the MK2 has ECU controlled alternator.

Yes it is a straight swop for a mk1 alternator.

Yes you can use an after market ECU. Megasquirt PNP will control the alternator on the mk2. If you build your own then it's an extra board.

Reverant on MX5 Nutz does some really good enhanced Megasquirt ecu's for MX5's that do this along with all sorts of other stuff inc launch, boost control etc.

I would just swop it for a mk1 alternator and be done with it.