Take it to a gunsmith. You did not say what is wrong with it.
100-300 USD
the serial number on your ruger P345 is located on the right side of the firearm directly beneath the slide. its a silver looking plate with an 8 digit number.
Moondog Industries gas fill O-rings from evike.com. Quick, easy, and inexpensive.
100-400 USD or so
100-400 USD or so depending on specifics
if you mean the air soft pistol then yes it can take both 0.12 and 0.43 plastic bb's
Magazine
The Ruger SR9 uses a proprietary magazine.
MidWay
You need the help of a gunsmith
neither, also it doesn't matter which one is "stronger", if you have a a high FPS (strong), you can't play on fields
Not recommended.