HARDCORE

Hi. I'm new hear and I have a few questions. I ride a DT 80 from 1981. It's a grate bike but I have some technical problems, like no spark, something banging in the gearbox, and wen I didn't have problems with that when I shift in the fifth gear at gaining full speed the bike loses power and the rpm keeps falling. Does someone know what can be the problem because I am losing my mind right now? :angry: