Buy a manwell & work your way through it. All pretty easy stuff but best to have an experienced friend walk you through it & remember comments in the manual like ‘remove bolts & separate the cases’ take a moment to read but can take an age to perform. Sidenote, soft faced hammer/rubber mallet & be gentle, never metal hammer & watch what you are hitting, put that screwdriver down.
In my younger years I have tried to replace RHS seals without splitting the cases but wasn’t successful so have grown to believe that you can waste a lot of time trying to save time, do it properly.
(& replace both seals while you are there)
Its time you replaced the rings & cleaned the powervalves anyway?