Q) Can I use compressed air instead of steam to blow this whistle?
A) Yes, you can: but don't expect it to sound the same, as steam rapidly expands when released, unlike air, producing different sound characteristics. You will also need an extremely high air flow volume.
Q) Are the replica whistles any good or just bad imitations?
A) It all depends where you buy them from. However the ones sold by Rizzoli Locomotive Works in the USA are an example of extremely good sound and quality.