The reservoir cover (part A) is not merely for aesthetics. (as mentioned by @Josh in his answer)
The reservoir cover's (part A in the drawing) purpose is to press down on the rubber reservoir seal (part B in the drawing). If the reservoir cover is broken it is not a problem as long as it remains stiff enough to properly press against the entire circumference of the rubber seal (part B). Depending on the damage you may be able to keep using the part, as long as the reservoir doesn't leak.
(Thank you for the image @jbuk, if you want me to remove it pls let me know)
If however the damage is severe enough to cause a significant reduction in stiffness of the cover and you've tried gluing it and it didn't work you will have to replace it. The correct part numbers have been mentioned in the comments by @ jbuk; "part numbers for the replacement are Y-6JK 98080 (for left hand lever) and Y-6JK 98090 (for right hand unit)"-jbuk )
As suggested by @Daniel R Hicks in a comment you could try gluing the reservoir cover if the damage by the crash isn't too severe.
From the looks of part B and from my experience part B has a flat top surface meaning you could quite easily replicate part A (the reservoir cover) by taking a piece of plate material (wood, metal, plastic) of approximate size, drilling holes for the 2 screws that hold it in place and clamp the plate down to create the seal. If the plate thickness is adequate it should seal just as well as the original part. and it is very cheap to make. I would suggest using approx 3-4mm wood or 1.5-2mm steel/alu and 4-5mm plastic for material thicknesses.
For making the outline i would suggest using a jigsaw or power jigsaw (with very fine teeth) for plastic and wood and an iron saw if going with metals. For rounding the edges either use an angle grinder with flap disc or metal/wood file.
It won't look perfect but it's a cheap solution.
I forgot to mention you'll have to countersink the screw holes on the bottom side of the plate in order to make room for the rods on the brake reservoir which hold the threads the reservoir cover screws screw into. In plastic/wood this is a relatively easy thing to do, in metal it might be a bit harder but still doable.
Alternatively you could drill a through hole of the rod's diameter and use washers on the top to keep the cover properly clamped onto the rubber seal.
I'll add a drawing of the part below:
Option 1: (countersunk hole)
Option 2: (through hole)
Note: part (B) rubber seal is not shown in the 2 drawings above, but it will be located in between the red and grey part as can be seen in shimano's exploded view drawing