For anyone who is interested: I replace one of the green troll heads, the right side one to be specific. Here was the procedure that I used, after lessons learned doing it.
1) I bought a troll head for $20 including shipping. Note that the troll heads have a thin border of troll plastic on the bottom fringe below the lower hole in the head. This is a weak spot that breaks fairly easily and should have been more robust. Anyway:
2) I put an object between the bottom end of the troll assembly solenoid plunger and the bracket underneath the playfield. This lifted the troll head assembly above the playfield so I could work on it.
3) I put a post sleeve underneath the trap door top of the troll assembly to get some clearance to the troll head.
4) I bought at Home Depot and used a 5/16 ignition wrench ($2.49) to get the 2 screws off. I also experimented with using a cut post sleeve between the back of the troll head and the bracket behind it to create a bigger gap to access the screws. This eventually I found out, was not needed, but some might get some help from doing that.
5) Slide the old troll head off and install the new one. Note that there is a black little flange plate with 2 holes cut in it. The screws go thru this little flange plate, which rests in the groove outline at the back of the troll head. Set the screws in the holes by hand and tighten.
Good luck, I hope this was helpful!