In looking at other 1.0L Fiesta motors, only 2 - the issue of that "hole" seems they don't need anything - as long as oil is not gushing out. Or seeing exhaust so if the hole is open but not sucking air - or bleeding out fluids, you're ok.
I've seen several 1.6L Fiestas that in some fashion - have a hollow in the cylinder head - almost as if it's for a fuel injection port - they are along that same axis as the "knock sensor" sensors and also follow a bolt (like 6mm x 1.25 pitch thread) on the 1.6L Fiestas..
So if it bothers you to have the hole open, you can put in a simple cap bolt of 5~10mm (height/length - just enough thread to cap that hole to seal it - nothing more) or so that fits and matches thread pitch - looks like in some fashion it is a "spare" for a bracket mount that may have been needed or designed in. Only it is not used in your configuration.
The thing I see in your motor is the "anodizing" or painting of the block - but this hole rusted because lack of paint used around that flange of the bolt.