Prototype info:
MPEX 5000 is is classed as a SD50-3. The unit was built from ATSF SD45B #5581 (ex ATSF SD45B 1881, Nee ATSF SD45B 5501, Not sure of original SD45 road number).
The unit was rebuilt by Morrison-Knudsen, with a left over cab from the failed MK5000C line. It is M-K's exclusive design of safety cab.
When built, it was rated at 5000 hp, same as the MK5000C's. The unit was delivered to DME as #5000. It was returned a few years later to M-K's sucessor, MPI, where it was assigned MPEX 5000. As far as I understand the unit has been de-rated to 3500hp.

Model info:
I got started on this project by starting modification on an EMD safety cab from an Athearn Genesis SD70M. The roof profile is generaly the same. The nise will have to be scratch built, and I intend to use my Micromarketcher to make new window frames, the battery box doors, and the nose door.
A month or so after I started the cab work, I saw a posting on a group I belong to, that a member (Mike Rose) was selling some units that had been previously owned by Brian Banna had owned, and started work on. Among these was a Proto 200 SD45 that had been stripped of its Santa Fe paint. Here was my chance to really get working!
The majority of the cutting work involved was to remove the battery boxes from the P2K SD45, and the nose from the Genesis cab. The Genesis cab will also have to be sortened length-wise (not completed yet).

ATSF 5501, Charlie Slater model, Josh Baakko photo, WPM 2008
Here's some refence material: