My vespa px 150 fell over now idling very high and gas gauge not working.

Two seperate problems there mate, your gas guage is probably not working because the sensor has become dislodged or damaged when you dropped it but they are so unreliable anyway I wouldn't be too concerned.

You will be idling too high for one of two reasons:
1. you have knocked your throttle slide out of place which is housed inside the carb housing, open up the carb top box and just make sure when you rev the scooter you see the slide move freely and to a fully closed position when the throttle is closed. Either the throttle arm is out of place which is attached to the slide or the housing itself has become dislodged. if this is the case just undo the screws attaching the carb casing to the body reset and re screw them but NOT TOO TIGHt, just enought to hold ion place and evenly tightened.

2. your slow throtle screw has moved a little. coming out of the middle of your air filter is a screw which will adjust your idling speed. Mark where the thread is now and then with a flat head screw driver turn the screw in half turns in the direction which eases the idling speed. Always count how many turns you have done as if this doesn't fix it turning one way you'll need to return the original position (ie so count back in half turns) so you can start again from the other way

Hope this helps :0)