Unfortunately, there is very little in the way of trouble shooting steps to be able to determine if it’s a bad main board or bad control/display.
If you are handy with a multi meter: check for voltage at the pins where the display connects to the board by disconnecting the display’s ribbon cable from the control board which will expose the 4 connecting pins on the main board and then seei if you have 18-24 volts when touching the meter’s leads to pins 1 and 3 with your system powered on (those pins being Hayward’s typical pwr source and voltage for its displays). No or low voltage would mean your main board is bad somewhere (e.g., bad,rectifier or maybe bad transformer; less likely it’s a blown 24v fuse keeping power from feeding those display connection pins). On the other hand, should you be able to determine through this step that adequate power is getting to the display at the point it connects to the board (at those pins), then it’s most likely a bad display. But that doesn’t necessarily mean purchase of a new $450 display cures things. That is, while your new display may show its alive (LED readout), it may still fail to properly respond due to remaining undetermined main board issues.
While this troubleshooting effort may well be somethimg you’re not prepared to undertake, it should help to underscore how vexing - and pricey - it can be to both find the problem and to resolve the display issue.
Regrettably, your system is no longer being made - and making matters worse, it was not a widely purchased Hayward system. This accounts for the scant number of listed eBay components available for your system. While A few complete systems can still be found . . .as well as a few $800 replacement main boards and $450–500 displays, it’s still a real investment gamble.
Given the foregoing, you risk adding cargo to a leaking boat should you engage in expensive piecemeal component purchases in attempts to keep your system alive. It would be a far wiser investment to channel your resources into purchase of an updated system (pricey as that will be).