Yes, ammonia is a theoretically good way to store hydrogen. It is a powerful fuel. I remember the fastest spaceplane ever flown was powered by ammonia and liquid oxygen. He took Neil Armstrong to Mach 5.7 and he lifted several astronauts above the Karman line.
But we would have to find a way to combine nitrogen with hydrogen at low temperature, by enzymatic catalysis. It's possible, bacteria do it in the roots of clover and alfalfa.
To burn ammonia in heat engines without producing too many nitrogen oxides, it is necessary to work at relatively low pressure, and it is therefore necessary that the oxygen and the ammonia remain in contact for a long time, and that the combustion either catalyzed by the re-injection of part of the burnt gases, and that the air is pre-heated before admission.
It is possible to do all of this at the same time.