If it's only for a short time, I think it unlikely that you'd damage it if you connect with poles reversed. All it will do is try to throw the solenoid in the opposite direction - you'd feel it buzzing and probably see the core pin twitch.
If the solenoid
has packed in, don't lose heart. I'm convinced that the EGR used on some Vauxhalls uses the same one. Don't forget that the Multipla JTD engine is used in several Vauxhalls and Alfas as well as other Fiat models. Have a look here:
https://www.fiatforum.com/multipla/326006-losing-power-smoking.html
Looking at my earlier photos in this thread, you can see that the solenoid and valve are held together by 4 screws on the same pitch circle, at 90 degree spacing. So it's easy enough to buy a Vauxhall one (I got a brand new one on ebay for £45, delivered), split it and reassemble in the orientation to suit the Multipla. Make sense? I bought it a while ago but haven't got around to trying out my theory.