The main concern I would highlight with the HST is the terrible performance of the lifeguards. I don't drive HSTs, but if there is one thing out of everything detailed in that report that would, personally, make me feel much safer, it's an upgrade to these, seemingly insignificant bits of metal.
In essence, if these were more resilient, there's half a chance this accident wouldn't have happened in the way it did.
Its also more than just the spectaculars. The lifeguards are very much the last line of defence against any obstruction on the track - from animals to masonry to vehicles. I would hope and expect to see modifications in order to bring them in line with the latest standards - or atleast to get them as close to modern spec as is possible.
I think other practical modifications would be simple changes to give the couplers more resilience and to investigate bogie retention.
In terms of seat belts/air bags - the report is very clear. The drivers cab, even if this was a meridian/voyager/turbostar, would have been destroyed. In lesser accidents they may help but in this case, there was only ever going to be one outcome.
Modifications to the life guards (primarily), but also to the couplers and bogies, likely could have prevented the train from leaving the tracks where it did, prevented carrying so much force into the bridge (and ultimately, the ground), and given a far more linear debris field.