If you have only broken the upper part, which I have marked A on the exploded view diagram, then I don't think that serves a functional purpose other than making the brake levers look nicer. Therefore, you should be able to continue to use the levers so long as part B is in place, but without the added thickness of part A, the screws (part C) may be too long to fit in the threaded holes in the lever body. Shimano lists these as being 'M3 x 8', and they appear to be Phillips pan-head machine screws. You'd want to buy some M3 x 6mm machine screws to replace them and be sure of not bottoming out the screws.
Alternatively, Shimano make available the entire reservoir cover assembly, (26) on the diagram, under the part numbers
Y-6JK 98080 for the right-hand assembly and
Y-6JK 98090 for the left hand assembly. You may be able to find an online stockist of these spares, or possibly ask your LBS to order them from Shimano. However, as M760 series Deore XT is relatively old, it is unlikely that any spares will be available direct, and you'll have to hope that shops have the parts already in stock.
The full exploded view diagram is available at https://www.paul-lange.de/support/shimano/explosionszeichnungen_archiv/ST//ST-M765.PDF