when you power on the machine and either push the left flipper button -or- press the credit button (if there are credits) the "anti cheat relay" will engage, stay energized and lights will come on.

those fuse holders look like the origonals and probably should be replaced , there should also be a fuse holder by the transformer

In the images I circled in green the slam switches ( or anti cheat switches )
the one I circled in red I think is the "coin bounce" switch and I think is supposed to have a nub sticking through the bottom of the cabinet to turn off the G.I. (like your turning off the machine)

on Beam (resized).jpg

onbeam door slam (resized).jpg

Yes, I would suppose 2" flipper bats not sure if Steve young has the entire flipper bat & shaft or not , I know for sure he has the plastic bats.

I cannot determine if there is supposed to be two bells or just one.

There are no backbox images on IPDB https://www.ipdb.org/machine.cgi?id=1715 and the bally parts manual is unclear to me if there should be a 2'nd bell or not https://online.fliphtml5.com/vrtyz/axun/#p=74

my Joust only has one bell mounted on the side of the head , I tried adding a 2'nd bell on the match unit like my The Wiggler and determined one didn't belong there. the parts manual I linked above does not say "Not used"

another note: it apperas you have the "lift out mounting board" and it appears to have not been used. https://www.ipdb.org/files/431/Bally_1966_Capersville_New_Lift_Out_Mounting_Board_Instructions.pdf

In your image it looks like your "move space ship" target is covered by a rubber

yeah he has the flipper shafts and pins, http://www.pbresource.com/flippers.html

do your mushroom bumpers have good 7/16" rubber rings? https://user.xmission.com/~daina/tips/pub/tip0117.html

Yeah here's a pic of a mushroom bumper on my The Wiggler , a little bit of a pain to install it so it'd be ok to wait til it's playing reliably but get the 4(?) you need on your parts list.
IMG_2264 (resized).JPG

I noticed on your "center target relay" there seems to be an unattached wire. also it's difficut to determine if the spring is attached or not.
Bally on beam center target loose (resized).jpg

I will allow others in the community to chime in on where they think it should attach.
Bally on beam center target schematic (resized).jpg

There is a video of the gameplay in the linked game for On Beam , it sounds as to only having one bell, there are other clicking sounds that I believe to be points scoring and the "00-90 match unit" moving.

While I was working on a friends Fireball that had no bells installed I kept a cow-bell on the glass I would ring by hand if I got the skill-shot there was a somewhat negative sensory reaction the first time they heard the crispness of the bells the first time they played it. A made backbox cover helped dampen the sound.

I still can't verify if a 2'nd bell is needed or not (they can be kinda pricey) Steve Young at Pinball Resource didn't have the 4" bell for the side he had Bally 3" bells that would go on the match unit. to have 2 different tones I was able to use a black Gottlieb bell on the side.

Oh , I didn;t know they made a dual wound coil that small now the schematic makes more sense to me.