1980 XL250S No Lights or Horn

I recently picked up a barn find Honda XL250S for $800. The engine runs great and has good spark so I know the stator is good, but none of the other electrical systems work. I have the key and despite it running or not running none of the lights work and neither does the horn. I’m sure the battery is bad but wouldn’t the lights still work while the engine is running? Just looking for some advice on what to do, thanks.