The fill valve on all modern toilets I'm familiar with has the supply enter from below the tank. The thing you need to check out is where the flange is in relation to the wall (called the rough-in). If you are lucky, it might be standard. If you choose a toilet with say a 10" rough-in on a standard 12" one in the floor, this would give you an extra 2" behind the toilet, and maybe give you enough room to run the supply hose to a new tank. So, measure where the flange is (it is measured from the finished wall to the center of the drain - do not include the baseboard trim). Then, you can see what may fit.
At what height is the supply coming out of the wall? and where in relation to the centerline?