I've used MV8114 to replace the transmitter/receiver led's. Places like Mouser and Digikey have them cheap. Marco sells them for more but if you already have an order going you won't have to pay for shipping from a second place. Little shop of games looks like the cheapest place to buy new boards. The led's in front of Kenny often get knocked loose and reflowing the solder will sometimes fix it.
https://littleshopofgames.com/shop/electronics/stern-opto-receiver-circuit-board-520-5083-01/
https://littleshopofgames.com/shop/electronics/sega-stern-opto-transmitter-board-520-5082-00/