Mini Mania are prepared to sell me just the plate. I'll order when I can get through to them, but thanks for the offer.  I'm located in Vancouver, Canada .. typical Fall weather today, and I'll be glad when I have wipers again :)  Thanks also for the note about the switch .. I haven't reached that point yet, but will bear it in mind when I do.

You might touch base with minimania, they might sell just the plate.
Where are you located? I may have a spare plate. or at least one you can look at.

When I went from a single speed motor to a two speed motor, I chose not to change to a two pole switch. Instead, I just connected the wire from the existing single pole switch to the hi-speed side of the wiper motor.

https://www.minimania.com/part/MMKT0700/Wiper-Motor-2-Speed-Conversion-Kit-For-Mk1-Mk2-Classic-Mini
 

I didn't even know that the wiper adapter plate existed.  Mini Mania do have a kit, but I only need the plate .. will try to see if I can locate one - new or used.  Many thanks for the tip!

Are you using the Minimania adapter plate?
I have used it on two cars.
Maybe they don't make it anymore
Found it, but it includes wiper boxes:
https://www.minimania.com/part/MMKT0700/Wiper-Motor-2-Speed-Conversion-Kit-For-Mk1-Mk2-Classic-Mini

I'm changing the wiper motor on my 1966 Aus Mini from the old style (square body) to the new (round body).  I have all the parts, and understand that the new motor is attached with a 'collar', rather than the old style with the three 'pegs'.  Looking for help/advice from someone who has done this .. is there anything in particular that I should be aware of before I start drilling holes?
Any assistance much appreciated!