There is a QH brand that are a wee bit wider to fit in the Mini S drums. There is a running argument that the drums are hopeless however if they are well maintained(with me they were adjusted every month)they will pull you up perfectly well. Just drive within the capability of the brakes you have...if not fade will become your friend and you will plough into something solid...
Look for the wider ones as they will do the job - I had them for 9 years of happy mini driving with no issues whatsoever. However I have now gone disc's and feel more confident in the stopping power I have. I chose to go unboosted as I like the pedal feel without the assistance. One day I will think about it but just couldnt be bothered right now - priority is to get Henry back on the road first.