Instead of blindly changing the IR diodes on the opto , why not look at it on your phone with the camera ? Most cameras on mobile phones will capture the light emitted by IR diodes. If the front facing camera doesn't work , the selfie camera most likely will because it almost never has an IR filter in the lens.

Unless your original boards have damage beyond repair (very doubtful) I would have them checked / repaired. Even if these boards are 30 years old, they were well enough built that if they receive the care they deserve they could last another 30..they are the easiest boards to work on and are very well documented. If you ever do replace them, put them up for sale, someone will pick them up to repair for sure.

Do you have a multi meter ? The power circuit for the different voltages is very easy to measure / test and anyone can learn the basics to test it. Those burned marks should be taken care of as early as possible as it will only cause more issues as time goes by. You asked about J210 earlier , this connector does bring the power to the CPU and to the switches / optos . As mentionned it's possible (and very likely) that the 12v is no longer high enough and causes that issue with the optos, because if the 12v is too low the OpAmp monitoring the signal will always perceive the signal so low that it registers as closed.

The fact that you are using a reset board point to the fact you've had power issues for a while. I don't know much about that board , but you'd think with the the amount of pinballs they manufactured over the years, they'd be able to build a 5V rail that doesn't require another board to make sure the most important voltage in the machine doesn't drop.. If you need that board it's because other parts of the machine are failing.

To test things further , you'll need a multi meter and a few pointers. The issue could very well be elsewhere and not power related directly, but unless the power is ruled out first , you waste your time looking elsewhere.

The only "visual" tests you can do is carefully look at the CPU board to make sure there isn't any battery acid damage on the board (that will surely cause all sorts of weird things with switches) and to remove the boards and to look behind important connectors like J101 , J210, to make sure the solder joints aren't cracked and broken, but if you don't have a soldering iron or don't know anyone who has one , you'll only confirm that the boards needs a bit of care.

If I were you , the money you're willing to spend on buying new boards should be put towards having those connectors changed first and have the power distribution board checked. Even with new boards , those connectors need changing. A proper repair is to change all parts of the connector , housing, crimps and pins. The basic things to test are easy and with help from people here you can easily learn more on how to keep your 3 pins top shape.