The regulator is usually very reliable, most faults can be seen visually, thay need a good clean contact from the body of the regulator back to chassis (batt neg)
The magneto coil that charges the battery also powers the light so if theres no battery or a duff one the voltage can rocket skywards...yeah maybe this can make a balls of the regulator maybe not...I dunno that but for now I'd get the battery in place