Are transistors interchangeable?
Would there be any issue in using any other NPN transistor?


If you are missng a screw, can you add any screw ?

Would it depend on how long is the screw ? What is its thread is? How large is the diameter ? What is the material ? 

Similarily, all electrical components have electrical characteristics and parameters. Different components can tolerate more voltages and currents than others. Others are set up for a particular application even though they are all part of the same family of components (capacitors, resistors, inductors, transistors, diodes).

So yes transistors are interchangeable, if the type (npn/pnp) and required specs match.


  What would I do if I could not find this transistor?


You compare the parameters for the transistor you can have access too, and compare it to the one in the you want to have. You keep searching until you can find one that can handle  it. Now, someone might have used an a transistor that was overkill for the project, and so knowing a bit about the circuit would help. If the current through a transistor was only 10mA and they have a part that can tolerate 1A, well, that's a bit much, and you can find a part that is more suited to what the circuit it. But if you do not have knowledge with how to analyse circuits, then you should probably match the component (to be on the safe side).