An unknown SOT23 package component with effectively no part number could contain just about anything unless you know the context in which it is being used. Without a part number to go on, your best bet is to try and determine its function in the circuit and try to work out what kind of component it might be. From there, it *might* be possible to find an equivalent part.
It might be a simple transistor, or it could be a custom IC, or even a small voltage regulator. If it is just a transistor, you may be able to determine what kind of transistor it is (BJT, MOSFET, JFET), and find a transistor to replace it with that would meet the necessary system specs.