Probably simple but how do you remove the rocker arm shafts to replace them ??

Remove the rocker arm tower, remove the tower retainer bolts (you'll see why). The rocker arm shaft is threaded, procure a long bolt, washer, and nut. I used an old "crap" socket that I ground one side out of to fit flush to the tower. Then put it all together you have a puller. Once you get them moving a little they will typically come right out.....