The oil burner is a more suitable recipient of the 16x25 mm micro speaker. But a small speaker can be packaged in the coal burning tender. In fact with the widespread use of sugarcube speakers being available, it would be worth using one in the tender and another in the boiler.
1) Remove the DC board and relocate it using the same screws and mounting bosses on the tender floor. The board rotates 90°, as shown below.
2) Fit the speaker to the enclosure and mount under the oil box.
3) Plug the 8 pin micro decoder into the DC board and remount the tender tank to the floor.