Given your bike has a 7-speed cassette nearly any mech will do. The Shimano DX RD-M650 would be the most correct retro part but every STX, LX, XT or XTR mech of the same time period would do the job, too.
Some pics of the mess with your pedal and crank arm would help to see what must be replaced and what is still useable.
I've never had a problem like yours with my handlebars. So i'm only guessing
what the problem might be.
Maybe the bar has a smaller diameter as your stem allows and you are missing the shims.
Maybe the threads on the stem and the screw are knackerd. Maybe the stem itself has seen some damage. So i'd try to measure both diameters first, then chang
e the screw on the stem, then try another bar or another stem.
The
Hope this helps.