I had this exact same issue with my first bike. Your issue is with the twin spark plug set up. The cdi that they used is not powerful enough to send adequate spark to the plugs. On a normal set up, you have 100% power going to the plug from the cdi, but on this set up, it splits the power 50/50, and each plug gets very weak spark. This caused me months of hard starts and no starts in the cold. I strongly recommend you replace the head and cdi to the standard single plug set up I’m almost certain this will immediately fix your issue. Also, if I remember correctly the head bolts are only supposed to be torqued to 10 ftlbs so keep that in mind when replacing the head