Since you have a long experience, you are already ahead of lot of people with no field experience. Since you have done your ITI, and are already experienced enough, you should definitely consider doing a diploma in mechanical of you can also enroll for engineering degree in mechanical branch. Since you want to do part time, I think you don't want to sacrifice your job. If that is the case, give some state level exams and select the best college that offers part time diploma. It will definitely help you in advancing in your career.

Best regards for your future!