If there is a clicking sound when pressing the starter button that is indicative of a bad starter relay, a fairly common problem problem with Aprilia scooters.
sounds like the starter to me, if you pull off your belt case you should be able to turn the engine from there to make sure its not siezed, if its pulling more than 5amps then the starter motor is likely siezed, dont keep trying it as this could result in the wiring overheating, 0.6 ohms is a decent reading for a ground connection, if you connect your volt meter + from the battery + connection to the starter + connection with volts selected try to start it, the reading you will get will be the voltage being lost between the battery and the starter it should be low, eg less than 1 volt dropped, if not look for poor connections.
or try this link.
How to Troubleshoot Your Piaggio
or this one.
piaggio x8 starting problems Google Search