that exhaust header leak will effectively derestrict the pipe and actually give you a 10 mph boost on topend.
the jet and holder missing is a prob. i have all the parts for the encarwi,
9 out of 10 a new screw that is a little longer will hold the intake on, they rarely strip all the way through.
they take round bing jets btw start at a 54, if you get a de restricted pipe take it to a 56 or 58, run a hi flow filter ditch the carb airbox.