First, get a nice strong magnet and pick through gravel below for that missing screw.
Regardless, take that cover off and bring it to a few local electrical supply houses (NOT big-box stores) and see if any of them sell that same PVC conduit body. They may be able to sell you a cover, but which brands they carry will vary. Obviously you want one that carries that brand of conduit body, so call ahead. It was a huge mistake to leave that rope dangling out of the gasket, as it wrecked the gasket.
The rope is in there for future pulling. Most people's tendency is to assume "the last person" was a fool. My rule is assume the last person did that thing for a reason, and you just haven't caught up with the reason yet. At some point in the future you will want something, and with any luck it'll be related and voila.
Expect that aged rope to be flimsy as heck. It will have been sitting in water for years. Don't pull wires with it - pull a new rope similar to it, then use that to pull the wires. The water isn't because of the missing screw. The water is because all underground wiring is 100% under water 100% of the time no matter how much we try to seal it, or that's the assumption we make. Trying to make conduit actually watertight is a lost cause, unless you go to industrial extremes like pressurizing it to 2 PSI. And there's no reason to bother; wet-rated wire such as THWN-2 or XHHW is rated for 100% water contact.
Also by the way, don't use cables in conduit. It's legal, just unnecessarily difficult. Use individual wires (that are wet-rated such as THWN-2 or XHHW). Besides, NM/Romex isn't wet rated).
Lastly, if that conduit is anything other than a northern exposure, paint it otherwise UV damage will tear it up. 1) Lightly sand it with a green Scotchbrite pad so the primer will stick, 2) brush an alkyd primer like Kilz Original or Rustoleum 7780 (use an 80 cent cheapie natural bristle brush, it won't clean with water, just throw it away), give it a month to cure, then 3) top with your house's latex paint.