The 45K76 is a darner as you know.I have changed them years ago,there is a lot of parts need to change it & some of the holes aren't drilled & tapped as they should be.It's a a lot of work for a machine that might sew just under 1/2" & you have the markings of the feed dog on the bottomside of the leather since it doesn't have a walking foot.
You can start with a pin 7/16" or 11mm long,it's might be alittle longer than what you need BUT you can always grind it off or bend the release bracket on the backside of the tension.
They are for the light & should be 6 volts,you should take the end cover off & see if these wires come off of the windings or are connected to the wiring block where the others connect,if it comes from the windings it's 6 volt for sure.
There is a screw in the shuttle that the bobbincase pivots on when you open it,once in a while it can loosen up & rub against the race & make noise.Also if you pull it out to check it make sure to tighten the 2-race screws all the way & back them off 1/2 turn so the race can float.
When you first start sewing do you hold on to the needle thread for the first 3-4 stitches & are you sure the upper tension is threaded correctly,looping underneath is either too loose or upper thread threaded wrong.Make sure the thread is pulled down in-between the discs .
Before you start turning any screws are you sure it's threaded correctly & do you hold on to the needle thread for the first 3-4 stitches? You also need to stop & start sewing the the thread take up lever(chrome piece with an eye you thread through) all the way @ the top of it's stroke.