Who says they do not already fit? I know at least one that does 
Hmmm..
Images of replacing (delicately storing the original of course) my A1000 motherboard with something like this.
A backplate should be no problem, as the 1000 has that slide in backplate..
Just need some adapters (floppy drive (eject button positioning might be tricky if you use a standard PC floppy), LEDs, Power, Keyboard (A1000 original of course, unless there is a replacement that fits nicely under the A1000 and is the same color scheme), joystick ports)...
Hmmm.. Depending on the fit, would a laptop IDE optical drive fit so it ejects where the expansion slot without cutting anything???
Then, all I'd need is a monitor that fits inside a 1084s case...

Hmmm....
desiv