It would be not to hard to calculate the amount of lead to equal the original and fill the space and glass over it. Cheapest source of lead would be old roof flashing from a building demolishers if you can find it or old batteries although that may need melting down. You would need to cast it into manageable sized blocks. Investigate any health hazards in doing this.
My late dad used to cast his own sinkers and we were often confronted with a large ladel of molten lead cooking on the garage floor with a kero blow torch firing away under it. Often wonder what health problems he was creating but it was the 60's and we sprayed DDT and Dehldrin around in those days like it was red cordial
By the way I think you have the earliest sail number so far . Ian have we heard of any advances on 12 ?