You can actually buy new plugs and sockets if you can discover what make and model they are.
Until a few days ago I assumed that was a very big “if” but then I found a fascinating resource on the miataturbo forum: https://www.miataturbo.net/electronics-122/miata-oem-aftermarket-connector-guide-99970/ where someone has collated a list of part numbers for several MX-5 plugs, and another poster on that forum mentioned the Aladdin’s cave which is Auto-Click UK.
I think the 4-wire lambda sensor connector is probably this one: https://www.auto-click.co.uk/174257-2 What do you think? Seems to match rhino’s photos. The keyway slots on the side seem to be in the right places. The matching socket lower down the page looks like a plausible match too.
I just bought some connectors from them which will allow me to make plug-and-play extension looms for refitting my old supercharger to my current car (for example relocating the throttle position sensor will use about a 3 foot extension cable instead of chopping and soldering like on my previous car).
Hope people find that useful.