Rather than remove the plastic stakes and have to replace them with machine
screws and nuts, try the following:
1. To disassemble, take a drill bit that's larger than the "melted over"
end
of the stake and *slowly* drill into the end of each stake, using the dimple
in its centre to guide the bit. Stop drilling when you reach the metal. This
should allow the plate to lift off the stakes.
2. Before reassembly, try and obtain some very small "self tapping" screws,
such as those that hold together small electronic gizmos. Some have heads
with large flanges, like "built in" washers. If the heads aren't large
enough, you'll need washers too. If you can't find the right screws, go
ahead with the machine screw method.
3. Using a drill bit small enough to make the right size hole for the
screws, drill into the end of each stake, using the depression made
previously by the large bit as a centring guide.
4. Reassemble the keyboard parts over the stakes and fasten with the screws.