You need to test for the display voltages at the power supply and then at the display. Do you have a multimeter. Common sys 11 failure is the high voltage at the power supply needs to be rebuilt.

There are reputable repair folks down in LOU. Someone down there can certainly point you in the right direction.

high voltage is easy to rebuild. you can get a cheap digital multimeter from harbor freight. youtube how to test voltages. you will love the dmm after you learn how to use it. very useful. it will take about 2 mins to test. then you can figure out how u want to spend your money in a new board for 100+ bucks or rebuild the high voltage for 5 bucks in parts

You can get the rubber piece for the helicopter from Steve @ PB Resource and fix that yourself pretty easily.

I'd start by removing all the display connectors and then put them back on , and at the other ends as well, see if there's possibly a bad connection.
Also Try the same thing with the display ribbon both ends, before worrying about bad power supply.
start simple first.

ribbon cable will cause this. Turn game off and remove and reseat it about 4 or 5 times ,it cleans the terminals and should make a good connection.
Also the 39k ohm reisitors I bet are out of spec in the HV section and they will also cause this.

Be careful as there is high voltage in there.

Do this on both ends of the display ribbon cable.

That whole +100VDC section of the HV from Q2 and ZR2 looks suspect. In particular Q2 and ZR2.

Yes, exactly what I suggested in post #2.

Alternatively, you could borrow a friend's working sys 11C power supply to check if your display is good.

You could always spring for a new board. They aren't that expensive.

https://www.marcospecialties.com/pinball-parts/D-12246

Or you could try your hand at board repair.

https://www.greatplainselectronics.com/proddetail.asp?prod=WAN-HVP-KIT

Marco board looks nice.
Finally a design with the appropriate HV design (pretty much same as original - the way it should be), switcher for 5V for nearly no heat generated with low voltage supply, name brand (Panasonic) capacitors and all heavy traces on bottom of board where they belong. Actually... looks like *all* traces are on bottom. Would have been a bit better if they used the top as a large ground plane for added heat dissipation. But overall - quite nice design.