For the UPS model Niky 900 by Daker, I'd like to replace the original CSB HR1234W F2 battery with something I can buy on amazon.it, e.g. FIAMM models: could you suggest what are the relevant parameters to match, in particular how do I compare the 34W datum for the above model with more common Ah specs? Is e.g. the model FIAMM FG 20721 (12V - 7,2 AH - 7AH) good enough? (I've made a 'translation' from W to Ah looking at this post) Should I opt for a 9Ah?
Yes, the 7Ah battery should work, but it will provide less energy than the original battery which is better suited for high power discharge.
The weird part about CSB model numbers is that for most series they show the voltage and capacity (GP1272 is 12V 7.2Ah), but for the HR and HRL series the model number shows the voltage and the power that a single cell can provide for 15 minutes. So, your HR1234W battery can provide 204W (34W/cell * 6 cells) for 15 minutes.
And it's even written on the battery too - 12V 34W/Cell/1.67V/15Min.
W is a unit for energy, while
Ah is a unit for charge capacity. You can't directly convert these units to each other. A low capacity battery can supply higher power if it has lower internal resistance.
And, I don't understand your hesitation for selecting a battery. I strongly believe that both batteries can supply 34W power without any problem at normal environment conditions. Purchase the highest capacity battery according to your budget, so that your device will run longer when there is a blackout.