Take pictures from many angles.
Loosen up the bolts which hold down the rocker arms.
Remove the entire rocker assembly.
Take some more pictures of it while it is on the bench.
Disassemble it being careful to organize the parts so they go back where they belong.
I use a wire wheel on a bench grinder to clean up the rust (and to scatter small parts into the deepest recesses of my shop
).
Check the rocker arm shafts for galling, and the rocker arm tips for notches where they contact the valve stems. The tips can be carefully polished smooth.
Wash parts in solvent without mixing up the parts.
Make sure to clean the crud out of the inside of the rocker arm shafts.
Reassemble.
