Script updated. Now vanilla mortars shoots flares that can be actually useful.
You can tweak its values in init.sqf, see demo mission.

al_mortar_flare_intensity = 80;
publicvariable "al_mortar_flare_intensity";

al_mortar_flare_range = 250;
publicvariable "al_mortar_flare_range";

You need to place this line inside init field of the mortar
this addEventHandler ["Fired",{private ["_al_flare"]; _al_flare = _this select 6;[[[_al_flare],"AL_flare_fix\al_flare_enhance.sqf"],"BIS_fnc_execVM",true,true] spawn BIS_fnc_MP;}];