That's the sensible way to do it. It's like how GM invented the Class 59, by squeezing what was basically an SD40-2 in a new bodyshell. The body is probably the easiest part.
It's actually a lot harder than you might think for a locomotive.
Here in Australia we have two locomotive manufacturers - United-Goninan Limited and DownerEDI Rail - who use GE and EMD drivetrain components respectively. Fitting the significantly larger components into a loading gauge which is similar to Britain's while also making it suitable for tougher Australian conditions is the toughest part of the whole process, which is why the expertise of UGL and DownerEDI is needed rather than ordering them directly from GE and EMD. When they engineer their drivetrains, they are thinking primarily about locos for the home market rather than reduced-gauge locos for Britain/Europe or licensed builders.
A good example of this is GE's Evolution Series drivetrain. It's a fantastic drivetrain (the AC version especially) but it is too tall to fit within an Australian gauged body, so UGL is still building locos using the older Dash 9 drivetrain.
The new PowerHaul drivetrain from GE will be a refreshing change to the normal way things go. Because it was first used in a double-cab British loco (the most restrictive configuration possible) there should be no challenge for European PowerHaul locomotives or license-built locos in places like Australia where you only need one cab.
For a freight loco maybe. For an EMU I'm not so sure
I would say it's actually the other way around, the challenge with the body of an EMU is only really to do with the design of the interior. What takes up the space in an EMU is the passenger accommodation space, the traction equipment being relatively simple to rework given the space required and the space available tend to stay pretty constant.
For a heavy freight locomotive the space is almost fully taken up by the traction equipment, so any change in the space available has a big impact.