import unreal

OLD = unreal.load_asset("/Game/ImportedAssets/Trees/SM_tree04.SM_tree04")
NEW = unreal.load_asset("/Game/ImportedAssets/Trees/SM_tree01.SM_tree01")

sub = unreal.get_editor_subsystem(unreal.EditorActorSubsystem)
actors = sub.get_all_level_actors()

old_mesh_count = 0

for actor in actors:
    comp = actor.get_component_by_class(unreal.StaticMeshComponent)
    if comp and comp.static_mesh == OLD:
        old_mesh_count += 1

print(f"Found {old_mesh_count} instances of the old mesh")