Aftermarket regulator GV125

Hi guys, just bought my wife a 2007 gv125, the regulator isn’t charging the battery enough, its reading about 12.8 with the lights on, at about 3500 revs, I’ve been looking around for a regulator, but they are all silly prices, has any one put one on from another make of bike, as these bikes are mostly replicas of popular well known bike, I was hoping some parts might be the same

Thanks in advance bry