Commando valve guide fitting

Have a read of this before you consider full guide replacement, K liner would be the way to go so you don't risk cracking the head around the guide.

https://www.nortonownersclub.org/node/11581

Agree with John

I have a friend who refurbishes Kawasaki heads and swears by k liners.

K liners are fitted by an engineering shop.

T & L engineering in Bedford do them, call them on 01234 352418 speak to Barry who is a mind of information, and will explain them to you, and will not place you under any obligation for him to do them. There is a write up on the cylinderheadshop.com 

Be aware T & L have a long turn round time, if you decide to use them.