2531.80 (1998) correct movement wanted

According to Omega, you need a caliber 1120.

https://www.omegawatches.com/en-au/watch-omega-seamaster-300-m-chronometer-25318000

Which looks like this:

https://watchbase.com/omega/caliber/omega-1120

I'd suggest you send the watch to an Omega Certified watchmaker with an Omega parts account and hope for the best, they may be able to get a replacement movement from Omega.

There are some used ones on eBay, but with the prices they are asking, for used movements that will need a service, I would recommend following the advice from @JimInOz above. Also if buying used avoid the earlier Cal. 1109 movements, as the Cal. 1120 fixed a few issues.

You can never put the correct movement it since the correct movement had a serial number on it which matched the one on the case. Perhaps Omega might be persuaded to replace and renumber the missing 1120 but I doubt it (their policy is to reject watches with missing serials) and even if they did it would incur a massive cost, perhaps exceeding the value of the finished watch. I can’t see how you can fix this at a sensible price. If you just want to get it working you could buy any old 1120 (or 1108,1109,1111) from eBay and use that. Or maybe even an ETA 2892 or clone as they are very similar but it’ll never be a fully original again and even this way won’t be cheap. It might be better to find hands that fit the movement you do have maybe. What exactly is it?

Do you have any idea of the reference number of the movement in the watch. Can you have a watchmaker remove the back. If someone swapped out the movement for something cheaper, it would likely be a swiss 2892 or a chinese 2892. I did some research and hands for the following movements are interchangeable. 1108 1109 1110 1111 1120 ETA 2892-2. All of them are based off of the 2892. I would consider the possibility that the watchmaker damaged the hands instead of the movement being wrong. It would be a a major PIA to put a movement in, other than those I listed above. I am not saying it is impossible, but it is very unlikely. If was me, the next step would be to post a picture of the movement with the rotor postiioned so the movement number is visiable. Look for a clover as well if it s Swiss ETA 2992. The other route would be to measure the holes on the hands. You have to see if they are warbled out. Ø hole size hour 1.50, minute 0.90. If you have any of the movements listed above, I would lean towards damaged hands. You would be better off having a watch maker with an omega parts account make this determination. I did find a Omega minute and hour hand online with a little searching. Link to Hand Seller

Some good points raised there. I forgot about the current Omega policies.
To offer any more helpful advice we need to know what has been shoved into the case.

Modding to fit a quartz movement wont be straight forward. The quartz and auto models are quite different. It may be possible to drop in a generic ETA movement (the 1538 used in the Bond quartz is a lightly modded ETA calibre) but the fitment is not the same as the auto and you may run into locating ring and hand fitment issues. The auto and quartz models are not identical.

I think you need to show us what is in there right now and we can offer better advice.

Then get it opened. You are basically wasting our time and yours at present