I wouldn't use a 75g w/out the center brace. The center brace is quite large on my newer Aqueon 75g. I'd determine what you have first. Is it an Aqueon/All-glass (Central Garden and Pet), or is it a Marineland (United Pet Group)? Go from there ordering a new top frame.
Not sure what brand but I have a LFS that can order me a new one for like $25. There's no hurry though as I don't have room for this one yet. I don't really have problem replacing the frame, i just love how clean the frameless ones look.