Klaus,
ANYTHING that will hold the retainer down long enough for you to get the
keepers in will do! Something like an open end wrench with a hole in the
handle, held down by a rocker stand bolt, for example. You then lift the end
of the wrench handle to press the retainer down. The biggest problem
is that the valve spring is pretty strong, so you may need a helper to hold
it while you put the keepers in.
I trust you did get a new valve stem seal....

The rope trick is a good deal to make sure the valve stays shut while you are
doing this. The compressor from Harbor Freight will PROBABLY work if it can
be adjusted to the right size (width).

The deal with the socket and the hammer is GREAT for taking it apart but not
for putting it together. I've done it many times amid a rain of springs,
retainers and keepers. The problem there is finding all those little keepers.
I almost always lose at least one. But then I have an adequate culch heap for
those eventualities.
--
George Downs Bartlesville, Heart of the USA!