Your risking the structural integrety of your tank by having the center brace removed. Tank manufacturers will use thinner glass because of the center brace helping hold the tank in shape. Having the brace removed puts extra pressure on the seals.
sells frames for certin types of tanks and you may be able to find a replacement frame there. If not your LFS
may be able to order you one. You will need to take as much water out of the tank as possible. Remove the old top frame. Often times breaking it into small bits as you do it. Clean the top glass from old silicon and then applying new silicon to the top and putting the new frame on the tank. Allow 24 hours to pass for the silicon to firm up before putting water back in the tank.