Step 1, figure out what parts are in the carb, step 2, figure out what is SUPPOSED to be in there. Step 3, determine what mods are done that may affect step 1. When the carb is off inspect the reeds for damage.
Clean the carb, replace the needle/seat and install the correct jets/needle/settings for your bike then see how it runs.
If, like others have said, you don't know how old the top-end is, then it is time to replace it.
The other side to this is I had an issue where my bike ran fine at idle and putting around to 1/3 throttle until I loaded the motor and hit the power band it would miss horribly. It turned out the plug cap to wire connection was old/corroded and dirty. I clipped 1/2 in off the wire to get some shiny wire and replaced the cap and away I went.