Do you have a repair manual? If so follow the steps for a tune up.
A tune up consist of several things. They should be done in this order.
#1 adjust cam chain tension
#2 adjust valves
#3 set points, gap and timing
#4 carbs
At this point you can skip down to points. If they are dirty or burnt they won't get the plugs to fire well. they make small files just for cleaning points. A bit of sandpaper folded in half will work. Once you get looking good you will have to adjust the gap. Often feeler gauges have a coat of oil as a rust preventive. This oil can get on the points. Once you set the gap use a strip of clean white paper, business cards work well. Dampen with electrical contact cleaner. Open the points, slip in the paper, let points close, pull out paper, repeat as need to remove any oil.
This should improve the spark.
Once you get good spark you need to insure it happens at the right time. For this you time the points. Your repair manual explains how.
If cleaning and gapping the points doesn't help you might have dirty connections somewhere between the battery and points. This often happens at the key switch and engine stop switch. If careful you can take the switch apart to clean the contacts. Cleaning any wiring connections along the way will help.
You might want to start at one end of the bike and work your way to the other cleaning and tightening every connection in the wiring. A bit of dielectric grease will help keep them clean.
Leo