Hi welcome to TBN. I am really surprised that someone has not responded to your question already. Usually there will be 2 or 3 knowledgable people responding to these kinds of questions.
I do not have a 147 and am not sure how the shafts are released, however I do know that Yanmar uses lots of spring roll pins. If this is what you are talking about then you need to carefully drive these out with an appropriately sized pin punch. You actually can get pin punches with a small nipple in the center to aid in centering the punch and holding it centered.
I take it that you already have the valve cover off and are wanting to get to the o-rings to replace them?
Maybe this will bump your request to the top and someone more knowledgeable than I can give you a better answer.
Mike