I replaced the needle bearings using sockets, and a vice.
Get a deep well socket that will fit into the opening and contact the needle bearing shell, then take another socket, with the open end toward the opposite needle bearing, place the whole assembly into a vice, and start tightening up the vise jaw.
I was able to press out the damaged bearing, and have the old still usable bearing take it's place, then install a new bearing in the empty spot.
I cooled the needle bearing in the freezer, and heated the rocker arm with a heat gun, I don't know if it made the job easier or not, but it wasn't that difficult of a job.
Put a bit of grease, or light oil into the area where the bearing will go to help the installation.
The new bearings should have a concave look to the flange area of the bearing shell, that's why the individual needles aren't likely to be dislodged during the time the rocker arm shaft is removed.
After installing the bearings, make sure they are slightly recessed from the end of the arm, if they aren't, you might not get the rocker arm installed, or worse yet the thin sheet metal shell isn't designed to take any loading, and it will crack out.