In the absence of fire-door specific guidance, I fitted the top hinge 160mm below the top of an FD30 rated door.
The manufacturer has now sent the guidance which indicates the centre of the top hinge should be within 100m of the top of the door!
The door is semi-solid core type, while the frame is mdf.
On another similar door I read that the old cutouts can be patched with hardwood and appropriate glue providing the dimensions are not greater than 110x36x3.5.
One slight issue here is the hinge cutouts go through the intumescent channel and take 2mm out beyond - to repair that I'd likely have to cut out a few more mm to give the patched in timber sufficient grab.