If you can see all the way through the boiler of your Aster Alishan Shay then you have a VERY serious problem. I've never seen a 'see-through' meth-fired model so far in over forty years.... However, on a more useful note, depending on the age and useage, your loco could just be plumb wore out - the first series are up to 47 years old now, tha' knaws. Even the second series are ten years old now, but gas-fired.
In the first series the entire three-cylinder motor block is a single die-casting, and they do wear out. Spares are virtually non-existent.
Sadly, it sounds liker a tear-down to me, and with that in mind, I recommend THE man - Geoff Calver at Rushford Barn, near Newmarket. He takes in, builds, repairs and deals in mostly Aster and UK-grown custom builders products. Give him website - Rushford Barn Models - a shake and make your number with him. Tell him I sent you. If you are not a member of G1MRA then you might consider it, but it's not a requirement of getting Geoff's services. He rebuild my Accucraft Royal Hudson after it was trashed by the TSA in San Francisco, and made a beautiful job. My Aster Br01 came from him, as well.