Is the power button the same as in the 27 inch

What broke? Take some pics so we can see.

Replacing the button is not trivial! It also rarely fails. Often I encounter people damaging the wires or connector.

The risk you have here is the length of the cable. I've not made a study on what they are so while the button part may fit and work you may encounter an issue getting the other end connected.

If you look at Step 28 in this guide iMac Intel 21,5" EMC 3068 Netzteil austauschen and compare it to Step 27 in this guide iMac Intel 27" Retina 5K Display Netzteil ersetzen you can see they are mounted at different points on the power supply and the speakers are different sizes so the distance from the edge (button) is also different.