Hi guys,the Front Brake Master Cylinder on my Classic has sprung a leak

,It isn't leaking from the filler so I assume it's from the piston?
Is there a repair kit?or will I have to replace the whole cylinder?
My Classic Parts Manuel has the part No for the round model,but sod's law dictates that my bike is fitted with square/oblong model

,so if someone has the part No for the latter please do tell.
Bearing in mind that the stock from A/N may be a tad long in the tooth

,does anyone know of a more modern bike that uses the same system?Thank you in advance,ride safe,J.B.