As a suggestion, You could use Half Life's VOX announcer (1998) for voicelines like John Ward's. You could add it as an option for the player, so you won't have to replace the original voicelines.
The VOX announcer seems like it has very limited capability that wouldn't be able to capture the variety of customised voice synthesis samples from the original game and further, I don't know if Valve would approve of that decision lol
I did suggest new synthesiser sources early in development to reflect the meta time period jump from 1980s to 1990s, but we soon decided that the original lines were too distinctive and iconic to 'Faith' as a whole.
I see. But as I said before: you could add the new voicelines, and from settings/options, let the player choose between the original voicelines and the new ones.
Besides, there's no need for the new voicelines to sound similar nor natural, since the original voicelines sounds very unnatural/robotic by themself. They just need the new ones to fit with 90s and Faith's atmosphere; I chose the VOX announcer as a suggestion for that reason, and I think Valve is reasonable enough to give you permission if you ask them (I can accept other alternatives, thought).