I just finished messing with one of these and boy was it a PIA. You are right, there is nothing to this game without a working helicopter, but boy is it a finicky mechanism. Probably the most finicky mech I've worked on on a pinball machine. The game I started with had nothing working too with it. The coupling for the lift motor was broken, the helicopter wasn't spinning, one of the limit optos was dead, the opto for the lift motor was dead, it was missing a mitre gear completely so wouldn't spin at all. It was a mess, starting at step one, so I learned a lot.

For whoever it might help, this is what I found to get the one I had reliably working:

I put on a new friction wheel and new pressure plate. If they are original, they pretty much all need them. I lightly sanded the pressure plate with 180 grit sandpaper to give it some "teeth".

I did put some grommets under two of the springs on the pressure plate. The two closest to the mountain mold on the left side of the cab.

The spinwheel that holds the friction wheel has to have the mitre gears set where the friction wheel pushes all the way in and doesn't have any lead room. Then, you want to get the set screw that holds the long rod of the helicopter mech tightened so that the friction wheel has just enough room to circle around the circular plastic. If it is too far out on the pressure plate, the weight of the mech will cause issues and the friction wheel will slip and not grip as well.

It is also imperative when tightening the friction wheel, to make sure the helicopter is 90 degrees off the rod at the end position where it takes the ball when tightening down. There can't be any give here, so when it spins, it gets the full rotation to 90 degrees.

All of this got the helicopter spinning around fine, however still not picking up the ball properly. There is a nut/washer on the coupling of the lift motor mechanism that you can raise up or down about a half an inch as needed that does adjust the timing of the lift motor slightly. For me, the helicopter and lift motor seemed to be adjusted fine, but the helicopter was bouncing into the mountain or knocking the ball off of its pedestal.

No amount of adjusting made it any better. I ended up moving the limit opto for the end position up a screw hole. So, the back screw went to the front screw spot and I screwed in a new screw into the top spot. This moved the limit opto forward about 1/8-1/4th an inch and the helicopter would stop perfectly over the ball, wait on it, and grab the ball.

Now, everything was working well, but then I noticed at certain points the heli blade would quit spinning and occasionally the ball would drop off of the magnet. The wiring they used in that loom was pretty flimsy and after 30 years of stress back and forth, it just all needs to be re-ran. It is a huge pain in the ass, but worthwhile, to pull all 4 wires out of the loom and re-tie them in somewhere under the mountain with new/better wiring. Once new wire is soldered in, re-run back through the loom (also a pain), put the zip ties where they need to go, make sure to leave some slack on the magnet wiring enough for the turn, and that did it for me.

The helicopter blade starts spinning with the sound and video sequence, swings out perfectly, stops directly over the ball, waits a second for the lift motor to get the ball to the magnet, it is perfectly aligned under the magnet, the magnet engages, grabs, swings it out and drops if you hit the flipper like it is supposed to, or delivers it to the left side ramp for multiball. I played about 20 games on it before it went back to the client and no drops, no issues, no questionable pickups, all perfect operation.