Change the whole engine. Not that mutch work.
By just changing the head you`ll probably be keeping the most tired bits. Shaving the head will increase the compression pressures and therby increase the blowby caused by worn rings & bores. Will also cause more strain on worn bearings .....
How mutch to shave off a head depends on what compression rate you are after, what type of 4k you got (twin squish?) , standard cam? , standard carb set up? , extractors ?, type of fuel you want to run ? .