I am sure you could adapt some other switch. 

I opened one of the OEM switches years ago.  I remember the plunger had an o-ring seal, presumably to keep oil from escaping the gearbox.  Therefore you may want to start with a switch that is sealed.  I suppose you could build a seal into the adapter instead.  Keep in mind that you will still need the spring to provide the correct amount of force as it is operating the reverse selector detent.

Ok so I checked the NZ source who showed them available on their web site. They indicate stock was wrong they have none and dont expect any. So I am thinking a threaded adapter to allow using the rod change switch. Has anyone done this?

Steve, yes, this switch would replace the reverse detent right below the oil filter.

When I checked on GAE131, no one had/has it in stock.  A year ago several vendors appear to have been selling the similar switch used on some TR models for their reverse and overdrive lockout switch.  That switch has a plunger ball nose that is too large to work with the Mini's reverse detent spring.

I believe Mur's suggestion about making an adapter for a modern switch is probably the best bet unless the OP can find a NOS switch somewhere.

Are we talking about a switch that replaces the shift detent on the front of the remote shift trannys? I'm not finding the part number you list. Steve (CTR)

I think an adapter could be made to use a back up lamps switch from a more modern transmission. Do you have the plug or the remains of the original switch? Also, these were on the ADO16 cars, so there will be several sitting on junky old transmissions that no one will ever get to. I am sure I have thrown many away over the years-switches and transmissions with switches on them. I do recall the switch interferes with using a large spin on filter.

Hopefully my picture link below will work.  A number of the switch parts are NLA.  I've got measurements of the various parts in the picture.  They should help you improvise the parts beyond the switch itself.

Thks willie that looks like the answer.

I need everything: switch, plunger and spring.

Zami what do you currently have?  What I am asking here is are you replacing a bad switch and have the other parts required for the switch to work (ie. the spring and steel plunger)?

//www.classicminispares.co.nz/store/switches/reverse-switch-remote-type

Trying to locate a reverse switch for a remote shift housing. I believe the p/n is gae131. Any out there for sale.