Erok7620, if this is an older 90 gal
, it might not need
a center brace. Older tanks were made with thicker glass. It wasn't till the manufacturers switched to a thinner glass that the center support became neccessary. If the top trim is intact and never had a brace, you are good. Also you can look at glass thickness. The tanks without a brace is thicker.
But, there is no reason you cannot add a center brace if you wish. It would make getting glass covers a lot easier!!!!!! If you want acrylic, try a local glass supplier. They will likely cut any size you want.
Good Luck with it, what are you planning to stock it with??