2013 TXC250R Backfiring, rough acceleration and power loss

Hello all,

My brother's bike stated having some engine issues when he rode it last time. Everything was fine up until the moment when positive wire that is attached to battery got loose, he felt power loss and eventually bike died, he was able to start it again using electric start and rode for few more minutes. then, when bike died one more time, he took off seat and found that wire is almost disconnected from battery. He pushed bike to our camp area and tightened wire back and since that time, bike starts well, idles well, but backfires and misfires when accelerating.
Any guess what the problem could it be?