These amounts will continue to be charged to both Italian and non-Italian operators however the significant change compared to the previous rule is that non-Italian non-commercial operators will be charged only if the aircraft stays more than 45 days on Italian territory instead of the previous limit of 48 hours. Non-Italian registered aircraft having spent more than 45 days on Italian territory will be subject to the tax on a pro-rata monthly basis, i.e. 1/12 of the annual rate for each month spent in Italy in excess of the 45 days tax exemption period.
This is a big improvement but the interesting bit is how exactly they will define "under maintenance" which I believe is an exemption provided for somewhere.
Will it mean that the aircraft will have to be "under" some Italian maintenance shop, if still grounded beyond the 45 day point? At most airports, you are not going to find a suitably competent maintenance facility so will probably be arranging someone to come over to fix it.
They are hardly going to permit the pilot or the owner to self-declare non-airworthiness as this would lead to obvious work-arounds.