I don't know the answer to that as I'm not familiar with the M23 power unit...However, you can get a liner from Westwood liners at a very reasonable cost if you give them the essential dimensions...
They can usually match to an existing liner (there are thousands) but if not they can produce to your specs.
You will have to take the dimensions from the barrel at the narrowest point...With the bore dimension (82mm-83.5mm depending on the current overbore) you can then work out the wall thickness of the casting..
BSA barrels are usually pretty generous with the material and fitting a 2mm wall thickness liner shouldn't present any problems and will comfortably give you the capacity to bore to +.060" in the future (if you live that long) but check carefully...
I generally don't fit liners with a thickness below 1.5mm unless I really have to..
The other relevant dimensions are length and the size and depth of the 'top hat' if you decide to fit a liner of that type..(as the M20 liner is)...
Bear in mind here that you have to retain a 'spigot' of the correct diameter and depth to fit into the head recess...Ian
email (option): ian@wright52.plus.com