Valve keeper installation without head removal

So I got this old DB1 that belonged to my sister years ago. I have already replaced belt/tensioner/pump and finally got it timed and the injectors washed. At idle, two of the rockers jumped, causing on of the keepers to fail. The valve did not make contact as far as I can tell. So here is the golden question.
How do you replace a keeper without removing the head?

So far I have been told,

  1. Snap on makes a tool for it.
  2. If you somehow fill the cylinder with air pressure to hold the valve, it can be done.
    (they never mention how this is done, just that it can be done)
    Also if there is any knowledge on why these rocker arms are jumping, it would be much appreciated. Thanks g2ic’ers

Old trick to keep your valves from falling in is to get some nylon rope, dont remember the size but it’ll need to be small enough diameter to get it into through the spark plug hole.

Position the piston to about half up of TDC and feed as much of the nylon rope you can through the plug hole, then rotate the crank so that the piston compresses the nylon rope. You will feel it alot of resistance as your compressing the piston just go slow and compress enough so that it adds enough pressure so that the valve wont fall.

I did this when i changed out the valve stem seals off my old EF sedan, the only specialty tool i used was a valve spring compressor. Just compress the valve spring and hit the retainer with a deep socket the valve keepers will just pop out just be sure to have a magnet to catch them to keep them from falling into the head oil ports.

Hope this helps