I don't know if data sheets exist for these, so let's work from first principles. The advantage is that you'll get to learn about your loco. Take small steps and never take anything apart that you can't get back together. If it is leaking steam, you may need to get some gasket material (often called 'gasket paper', but it's more substantial than that, and not paper-based, either) and some paper punches; and then make your own. O-rings should be nitrile, I believe, and will be either obvious Imperial or obvious metric sizes. If you haven't got a micrometer or vernier callipers yet, I would get one: digital verniers are comparatively cheap and simple to read. Taking locomotives apart, and reassembling them, isn't difficult, but you do need to be careful! Don't force anything; threads are very fine and can strip easily! Some will have threadlock on them: this can usually be softened with gentle heat (cook shops sell small blowtorches for caramelising food, which are ideal for this. Always work on a tray, with a towel laid on it: small parts can travel a long way when dropped... How do I know this?
Sorry for the long post!