Thanks. I haven't the time right now to get to the engine(assembling linkages on a Hiperbipe build). But it appears from the links you've given that the top valve spring retainer (doughnut looking part) is pushed down the valve stem far enough to remove the two half sleeve parts. Then the round retainer and spring can be removed and replaced. And I do have some clear gallon bags.

Thanks for the answer. And it is curious that the operator's manual that I received for the motor from Tower about six months ago has no parts diagram.