The only issue is these older HSTs have a different engine and other things along those lines, so it's not as easy as a reskin.
Then there is also the Intercity Executive livery, which looks like the Swallow livery, but is slightly prettier in my opionion, because it uses more of the original shapes (the yellow wraps around the front and continues onto the sides)
This one is a buffered unit also https://forums.dovetailgames.com/attachments/penzance-_-_geograph-org-uk_-_102199-jpg.52223/
You would need a new model for the original build stuff. The cab layout is very different to the current TSW version and it also had a different power unit. Also for later periods (late 1980s) the Western Region power cars were fitted with ATP. Also don't forget the trailers were fitted with "Central Door Locking" from the mid 1990s. Also if you go to as built they didn't have TGS and the power car had an additional window in the guard area which was overlooked in Armstrong Powerhouses EP pack. As ItsYa165 also pointed out in late BR early Virgin days you have the power cars used as DVT's on the ECML electrification that went back into the HST pool and use until removed form service.
This is the early Western Intercity Franchise livery before First group took over I believe. Known as the "Merlin Livery" The other scheme you have illustrated is a copy of the First service livery by British Rail.
An original HST model as built would be a great addition to TSW alongside a suitable late 1970s route. Paddington to Reading Circa 1970s would work well. "Diesel Legends" would fit in alongside more loco DMMU addons. Its the perfect route.
My dream add on pack right there. IC Swallow and Great Western Merlin liveries with amazing Valenta screams. They could source the sounds from Armstrong Powerhouse again or record them from the Prototype......but I don’t think that is running at the moment?
I would say they would do well to get AP onboard for sounds. The modelling of locomotives lately for TSW2 is quite good. From watching the railfan fest at the weekend, one would assume they have quite clever people on the sounds and horns apart the sounds are quite good in TSW2. The Clinchfield Route is epic xD. I think the issue with sound is everyone has a different opinion, and we are only human and hear sounds in differing ways. As long as the sounds are collected and put together correctly I cant see their being an issue. (as long as there is no clear looping of sounds).