that ball in the 'cage' doesn't look to be original...

does it move if you try and roll it by hand?

it's no big deal if it's there or not, as mentioned.. it's there if a player lifted the machine from the front it would cause the ball to roll towards the rear of the cage making contact with the switch and causing the machine to 'TILT'.

it looks like it's an original playfield ball and is touching the top of the cage stopping it from moving....again no big deal.

as you probably know already? the front door has been modded to work (adding credits) by pushing the added on button above the coin return cut out.
I think you will struggle to buy new replacement parts and may be better off seeing if someone has a complete 2nd hand door and if your budget allows, put on a new door skin?