I want to make Axotols spawn on water without needing clay. This is my code:

@Mod.EventBusSubscriber(modid = "aoemod", bus = Mod.EventBusSubscriber.Bus.FORGE)
public class SpawnConditionsEventHandler {
    @SubscribeEvent
    public static void spawnConditionChanges(LivingSpawnEvent.CheckSpawn event){
        LivingEntity entity=event.getEntityLiving();
        if(entity instanceof Axolotl){
            BlockPos pos=new BlockPos(event.getX(),event.getY(),event.getZ());
            BlockState state=event.getWorld().getBlockState(pos);
            if(state.is(Blocks.WATER)&&state.getValue(BlockStateProperties.LEVEL)==8){
                event.setResult(Event.Result.ALLOW);
            }
        }
    }
}

I created a breakpoint under the first if statement. If the world doesn't contain any clay, the breakpoint never fires but if there is clay then the event fires. Essentially the problem is that I think I have the wrong event because the event only runs after the vanilla condition of needing clay is met.

The line of code that causes the issue is 

register(EntityType.AXOLOTL, SpawnPlacements.Type.IN_WATER, Heightmap.Types.MOTION_BLOCKING_NO_LEAVES, Axolotl::checkAxolotlSpawnRules);

Which references the method 

   public static boolean checkAxolotlSpawnRules(EntityType<? extends LivingEntity> p_186250_, ServerLevelAccessor p_186251_, MobSpawnType p_186252_, BlockPos p_186253_, Random p_186254_) {
      return p_186251_.getBlockState(p_186253_.below()).is(BlockTags.AXOLOTLS_SPAWNABLE_ON);
   }

I found that the method I am interested in is 

   private static boolean isValidSpawnPostitionForType(ServerLevel pLevel, MobCategory pCategory, StructureFeatureManager pStructureManager, ChunkGenerator pGenerator, MobSpawnSettings.SpawnerData pData, BlockPos.MutableBlockPos pPos, double pDistance) {
      EntityType<?> entitytype = pData.type;
      if (entitytype.getCategory() == MobCategory.MISC) {
         return false;
      } else if (!entitytype.canSpawnFarFromPlayer() && pDistance > (double)(entitytype.getCategory().getDespawnDistance() * entitytype.getCategory().getDespawnDistance())) {
         return false;
      } else if (entitytype.canSummon() && canSpawnMobAt(pLevel, pStructureManager, pGenerator, pCategory, pData, pPos)) {
         SpawnPlacements.Type spawnplacements$type = SpawnPlacements.getPlacementType(entitytype);
         if (!isSpawnPositionOk(spawnplacements$type, pLevel, pPos, entitytype)) {
            return false;
         } else if (!SpawnPlacements.checkSpawnRules(entitytype, pLevel, MobSpawnType.NATURAL, pPos, pLevel.random)) {
            return false;
         } else {
            return pLevel.noCollision(entitytype.getAABB((double)pPos.getX() + 0.5D, (double)pPos.getY(), (double)pPos.getZ() + 0.5D));
         }
      } else {
         return false;
      }
   }

The problem is that it checks isSpawnPositionOk before checking checkSpawnRules which is where the event is fired.

I found the method 

   public static <T extends Mob> void register(EntityType<T> pEntityType, SpawnPlacements.Type pDecoratorType, Heightmap.Types pHeightMapType, SpawnPlacements.SpawnPredicate<T> pDecoratorPredicate) {
      SpawnPlacements.Data spawnplacements$data = DATA_BY_TYPE.put(pEntityType, new SpawnPlacements.Data(pHeightMapType, pDecoratorType, pDecoratorPredicate));
      if (spawnplacements$data != null) {
         throw new IllegalStateException("Duplicate registration for type " + Registry.ENTITY_TYPE.getKey(pEntityType));
      }
   }

Which refers to DATA_BY_TYPE.

private static final Map<EntityType<?>, SpawnPlacements.Data> DATA_BY_TYPE = Maps.newHashMap();

The register method itself cannot be used to replace the Axolotl spawn condition because of the IllegalStateException but I could use reflection to access DATA_BY_TYPE and then call the put method with my custom method (which always returns true) in place of checkAxolotlSpawnRules. The problem is that I don't know where/when to use the reflection because the reflection must occur after  

register(EntityType.AXOLOTL, SpawnPlacements.Type.IN_WATER, Heightmap.Types.MOTION_BLOCKING_NO_LEAVES, Axolotl::checkAxolotlSpawnRules);

Which occurs as part of the static{...} of the SpawnPlacements class