Yeah, I'd not want to use a PLC input for an overspeed event either.
Using AC frequency isn't always a good idea either, as you can conceivably have an overspeed event unable to be detected if the voltage goes below the detection threshold (i.e. loss of excitation and subsequent loss of load causing overspeed as an example). It also doesn't work for detection of engine at cranking speed, if the excitation is disabled below a certain frequency.
You can get discrete over / under speed detection relays, I'm not a fan of them but they're more effective than relying on a PLC. My preference is to feed the magnetic pickup into a dedicated generator set controller (Deif, Woodward, Comap etc) and use that for your speed protection, along with everything else the controller does. Crompton make some discrete relays that are suitable for speed protection if a cheaper option is desired.