Do you own and know how to use a multi-meter, have the electrical schematic diagram and service manual and know how to read it all or need any assistance with that part? Because electrical trouble-shooting motorcycles all starts there. and yes you would start from the source, so that would be you stator output. OEM service manual makes it super easy and It's all interesting fun stuff, if it's not cutting into your riding season.
Horn does draw a lot of power so that is an indication that at least that has a decent ground and battery feed. With a meter or even better an oscilloscope
you could literally put a value on how good every part or assembly is working.
cool ! will be very expensive if you need to replace the 4-pipe exhaust.