yea wow that's a lot of different problems you incompletely describe...
leaking from the decomp I suspect you mean the little hole on the underside of the cylinder next to the exhaust. This leaks if your decomp valve leaks, and is a common problem. solution explained here:
Knocking could be anything, be more specific as to where. could be the
if it's leaking gas from the tank, it's probably the petcock seals. easily replaced, just get a new petcock from doscycles or treats, or diassemble the one you got and tryn reassemble with new seals (replace in kind is best and easiest)
if it's leaking gas from the carb, chances are it's the float bowl s4eal. it's very common for those to pop out and leak on the clones. You can find replacement gaskets fr the float bowl, or just shut off the gas when not riding
Transmission fluid can leak from a few places but 3 are most likely. 1 is the clutch cover. if you took the 5 bolt cover off the clutch ever, and didn't replace the gasket with a new one, it's leaking there.
If you had to reall torque on the drain plug screw to fill the transmission and wrecked the threads or punched a small hole through, it will leak.
the third one's the pedal shaft seal. they wear, especially if you have or had a loose chain it can snag that and wreck the output seal on the pedal shaft. measure seal size and replace it with a new one. you can do this just pulling the old seal out once the crank pedal arm is off, and pushing a new one in.