Going by the 385s and 800s I'd expect something like this
View attachment 151247
Please note: very unofficial and based on speculative discussion. Take with a heavy grain of salt.
Engines would need to be on the driving vehicles or Hitachi would be severely underpowered on a 3-car unit. Though I'd be concerned about how much power Hitachi can get underneath, the MTU 1800 engines (used on the 195s etc.) are max 523hp which isn't a lot split between 2 cars. 769s had about that much power and they perform as well as a class 150, I'd expect GWR to want something a bit better.
While a battery is a useful addition I'm doubtful it would make up for the lack of diesel power. Hybrid systems which use the battery as a boost for acceleration could work but they'd need somewhere to recharge, either a long distance of coasting to use the diesel generators (Cornish Mainline isn't known for being flat), using the diesel generators/shore supplies at the terminus (former is very loud, latter is doable if inconvenient) or a big enough battery to last the day (needs a shore supply to recharge wherever the unit is stabled).
Hitachi could fit the bigger engines on the 800/802s but they'd end up with the high floors of the 800/802s intermediate cars which isn't ideal. I'd be very surprised if Hitachi has squeezed in multiple engines (the 230s managed that but struggle to cool them) or got them roof-mounted.
Stadler have a proven low floor platform with enough space for generators. Alstom (from Bombardier) and Siemens have inside frame motored bogeys, which are about 2-2.5 ton per bogey lighter than Hitachi's outside frame powered bogeys. Aside from UK manufacturing I can't see Hitachi's advantages.