You might also check with your LFS on a quote for a whole new top ring if it is the factory plastic piece.
I replaced the one on my 72g bowfront a year ago after a crack formed, it was a special order and was under $75 threw Blue Sierra. for a 90g standard tank, I'm sure it would be a much cheaper part to replace.
You just use razor blades to remove the old one and the silicone that held it on and then just put new cilicone in the glass channel of the new onwe and press it on then let cure for a day or 2.
Only other option would be to eurobrace the tank with glass or as mentioned, if the crack is not on the edge, reinforce it with acrylic sheets and bolts although I would go with nylon nuts and bolts.
Good luck, and hope this helps.
Casey