Sorry, couldn't find a link to that Barchetta/356 for sale anywhere.
I've never needed an IVA so hadn't really looked at the implications, but I just spent ten minutes looking for the bit about not cutting the 'screen frame out of a monocoque and it isn't in the IVA manual.
It is in the 'Radically altered vehicle' guidance, though, in as much as it says you'll need an IVA unless the original (or new oe replacement) chassis, monocoque bodyshell or frame is used unmodified.
What the IVA bit does say, though, is the following:
The condition/absence of an item in isolation is not a reason for an item failing to meet the requirements. However if the condition of an item
is such that a meaningful assessment cannot be made, then the IVA 30 should indicate that the applicable section/area was unable to be
assessed and state the reason for this action. Examiners are not required to carry out a roadworthiness inspection but where obvious safety
defects are noted the vehicle may be subject to prohibition action, The IVA certificate (IAC) will not be issued.
Where an item appears to be missing an assessment should be made of the effect the missing item will have on the suitability for use of the
vehicle and the safety of the occupants, pedestrians and other road users. Where a missing item is considered to have a detrimental effect
the IVA 30 (Refusal to Issue IAC) will indicate that either the item was unable to be assessed or that a required standard from the section
‘General Construction’ has not been met.
To me, that reads that the inspector has to make a safety assessment of anything that's missing or modified, so I would expect that chopping off the windscreen frame is fine so long as you replace it with a decent substructure to maintain the vehicle's integrity and safety.
Edit - just read a bit further and found the 'general construction' guidance on page 269. Well worth a look imho.
https://assets.publishing.service.go...r-vehicles.pdf
Please feel free to disagree with that view if you know better...