The electronic advance engines weighs 4 lbs 1 oz, the mechanical weighs 4 lbs 8 oz, hardly just a few grams...The difference is in the hub....
I'm not sure why you need the heaver hub? If I converted a Brison from mechanical to electronic, one of thredded rods would be eliminated, the ring would be fixed and a syncro module would be added. Little or No weight difference. In fact Brison reciently went from mechanical to electronic and there is no advertized weight difference. Are you leaving the original magnito hub with the large magnets in your mechanical set-up? Why do you need a different hub?