Not relevant to OPs specific question (because of the age) but a former colleague brought a Cooper Works for his wife around 2005 iirc.
He was not impressed with the build quality and showed us how the rubber window seals were disintegrating...and the car was only around 6 months yo a year old at that point.
Great fun
Mid 70s i had a mini countryman, with an 1100 engine and 12 inch wheels (among other bits). It was fast.
A friends sister had a mini saloon fitted with a 1300, no idea if any other mods but it was a classic sleeper look barring the wider 10s she had on. I drove it one day...bloody thing had so much power you couldn't pull away quickly without the front end literally bouncing as it tried to get traction