sd75i - The ES7 is my version of Dash 7 to Gevo-engined conversion. The idea actually comes from two pieces of information from GE. I also follow prototype railroading. (I have been a Railfan for as long as I have been a modeler)
A couple of years ago, GE was promoting the upgrading of older units. The conversion was referred to as "modernizing" locomotives. Which they did do to some locos of a railroad in South Russia.
Not unlike the old "Super7 Series" rebuilds. But, more of a "do it yourself" project. The Super7s were completed locos coming from GE.
In the "moderization: A good portion of the loco is rebuilt/reused. With only the Primemover and related parts replaced new. The assembly is mated to the loco and new sheet metal is fabricated to complete the conversion.
The second piece is from a post to Yahoo Group, Loconotes. A thread posted GE's creation of a "6-cylinder" GEVO Primemover a GEVO engine cut in half down the middle. The engine was stated to be for the Regional and Switcher loco power markets coming in at 2300 HP. An accompanying graphic showed the engine in a “B-trucked” loco.
The B23-7 was a 2300hp unit (2250hp). It was a no-brainer after that.
The modeling part is shown in the photo below.

So, take One BXX-7 body assembly. Toss out the long hood. Add an Dash8-32BHW long hood and an some styrene pieces and you have a FCN “B23ES7”
Again, you could look at it as being a modern day version of GE's Super 7 rebuilds. Which got me into doing kitbashes and scratch building in the first place, some twenty years ago
Jerry G.