Valve guide seal replace with head still on.
I cobbled some tools together for the task. See pic one.
Bring the piston for the cylinder being worked on to TDC by turning the crank, then back the piston back down in the bore a little ways.
Pack the cylinder with small rope as shown in pic two. (This happens to be window curtain pull rope I found in my garage.) You feed into the cylinder 6-8 inches of the rope end (recall that the compression chamber for the 550 is only about 10cc of volume and you just want to fill that), then bring the piston back up to near TDC, which mashes the rope between piston and the valve so the valve stem shouldn't move while the spring is off. (simple test)
Lever the spring down (compress), then remove the keepers from the stem with a small magnet. Move the spring off the valve to access the valve stem seal. Pry off and press the new one on.
Reposition the spring and retainer. Compress the valve spring again and re-install the keepers, with forceps or bent needle nose pliers.
Release spring compression pressure noting that the keeper have returned to original (proper) position.
After doing the other cylinder's valve, back the piston down in the bore and pull the rope out of the spark plug hole.
Move on to the next one...