System 11 coin doors are the same as the ones used on WMS video games like Joust and Robotron. You can always try and source a nice one from there, they seemed to take less of a beating on vid cabinets than pins.

Check to make sure all your switches are working properly. The engine revving is the flipper EOS switches and the tire screech is the shooter lane if I remember correctly.

There's actually 2 switches in the EOS switch stack. The one you're seeing sparking is the high voltage switch and the sparking is normal if the flippers don't have the suppression capacitors installed. It's the other switch in the stack you need to look at.

I think the main benefit to using the WPC flipper mechs is to get rid of the cone style flipper return springs. It's more expensive than a rebuild kit and honestly I don't feel it's worth the extra money for a machine that's only going to see home use. If you get a flipper rebuild kit they typically include the capacitors in the kit for you to add. The benefit to the caps is just to slow the pitting and wear on the EOS switches to make them last longer.

It always amazes me seeing wiring hacks like those, especially the GI ones. It take so much more effort to fix it wrong than it does to fix it right in most cases.

Is it just an optical illusion, or is the new target plastic smaller?

Honestly, I wouldn't bother with replacing the GI wire with insulated wire. A lot of work for zero gain, especially for something that no one but you will ever see. As for the cables on the displays, new ones are included for P3-4 and the credit match because the new LED displays all use the same number of pins for the cables as the original Alpha-numeric displays. Using your original cables for those two displays keeps costs down.