Recessed door jamb switch

Our house is 65 years old. The recessed door jamb switch at the top of the stairs to the basement stopped working a while back. Today I purchased a replacement, but the metal box is too large for the existing hole. Yes, I could chisel out a larger hole but I am not sure I have enough door jamb to accommodate the box due to the style of the woodwork.

I have searched for a smaller size, 14/16 inch width for box, rather than 1 1/2 inch.

Maybe it would be simpler to just connect the "interrupter wires" and replace the whole light fixture with a motion detecting light fixture. BUT, it seems like I would still need the metal box for the connected interrupter wires.

What are your thoughts on this?