I also used to keep seal kits around.  However, experiences in the non-Mini world made me stop that.  I also have owned Triumphs.  I found that  aftermarket cylinders for those cars often use different size seals than the factory cylinders.  Since outwardly you cannot tell the factory and aftermarket cylinders apart it no longer made sense for me to keep seal kits around... they may fit or not. 

I don't know if aftermarket Mini wheel cylinders use different seals than factory parts but I have been burnt before with the Triumphs.  It made more sense to start keeping complete cylinders if I kept any spares at all.  
 

Honing Master Cylnders i would agree with Doug== Its more of a pain, and i have been more unsuccessful in their rehab. the hone dosen't seem to leave the bore smooth enough and the seals in my experience dont work well.

For Brake Cylnders + Clutch Slave the hone does the trick.   Sure cost wise it is very effective to install new Cylnders (clutch slaves however have seem to gone up in price) in past months.   But in my opinion  the Brake Cylnders are easily Honed+ cleaned and put back in service with seal kits. Nothaving to deal with the hard line or the Spring clip on the back plate is a win.

Seal kits are coming a bit more rare but you can still find them... I keep them around as a good option.

Disassemble the master and give it a thorough cleaning.  Then inspect the bore for pitting.  If any pitting is seen, chances are you will not be able to restore a smooth bore and the master's seals may never work properly.  

Before I gave up on honing what I would do to service small bores was to buy a length of hardwood dowel the next size under the bore.  I would cut off a piece about 8 inches long and make a hacksaw split about an inch long down one end.  The other end I would whittle down to about 3/8" so I could hold it in the chuck of a low-speed drill.  I would take 320 grit wet/dry paper and tear strips about 1" wide and place one in the saw split of the dowel.  As I would wrap the paper around the dowel I would feed in a layer of paper towel cut into strips.  That would give the paper a bit of spring.  With the bore wet with light oil I would use a low-speed drill to run the homemade hone in the bore.  Do NOT stop in one place, you have to keep the paper moving up and down the length of the bore as the dowel/paper turns.  Move on to finer paper if you want.  However, as I mentioned above, any pitting left in the bore will shorten seal life.  Sanding deep pitting away will result in an oversized bore where the seals won't seat properly.  Remember that brakes are for safety and don't gamble.

I'm actually thinking on honing master cylinders. Any thoughts on this?

I have not honed a slave cylinder in years.  New wheel cylinders only cost a little more than seal kits.

CTA Tools 1120 Small Brake Cylinder Hone

https://www.amazon.com/CTA-Tools-1120-Small-Cylinder/dp/B0035533GU/ref=sr_1_1?keywords=small+hone&qid=1550023368&s=gateway&sr=8-1

I will not say this is the best quality hone but  it works.    Has plastic parts all over. Does get the job done.

What are people using to hone brake and clutch cylinders? I can't find much out there that'll fit 5/8"??