To fix BMX handlebars, you might need to bend the handlebars back to their correct position. You might also need to tighten the bolt that anchors the handlebars to the bike. Fixes will vary depending on what is wrong with the bike.
A BMX stem is what connects the handlebars and forks to the frame.
yes
Yes, all handlebars have the same diameter out by the grips.
Tighten the stem
Replace them.
you can searh online on skatehut , winstanleys bmx and dogg scooters are the best.
you probably want to replace them. They can be pretty cheap to buy off the internet.
the fork gyro (if have) and stem and mostly important the headset
It´s the Stem, it holds the bars and match with the frame with the help of a headset.
check for any obstructions on the frame if there is anything get it off
purchase a new set
it depends on what the purpose for riding your fixie is...for tricks use a riser handlebar because it allows for the best leverage when doing wheelies or keospins. you should use straight bars when your just cruising