Yes, in principle it can be repaired, but for a Canon 450D it is usually not worth the cost or effort unless you can source parts cheaply and do the work yourself.

The missing focusing screen affects the viewfinder image and can affect metering, but it does not cause normal viewfinder autofocus to fail. The missing submirror is the key autofocus problem: phase-detect AF relies on that mirror assembly to direct light to the AF sensor. Without it, AF through the viewfinder will not work correctly.

Replacing the focusing screen is relatively straightforward. Replacing the submirror is much more intricate, and proper alignment matters. For an older, lower-value body like the 450D, professional repair is unlikely to be economical.

Most practical option: use the camera for parts, or find a donor 450D body with intact mirror parts if you want to attempt a DIY repair.