Longer carbody

...to accomodate the steam generators in this case (I think they were officially pre-Head End Power).

The question is, why would you want to bash one when Athearn is selling both of them right now? It would not be an easy conversion, since I believe it would require lengthening the frame, among other things.

why go the easy route?

Well, in a normal situation, I would buy the Athern locomotive (the F-45 and FP-45, both undecorated) and do what I intend, detail and custom-paint the units (If all goes well, I would like to have a model of every F-45 and FP-45 made, as well as my railroad's own "Ghost fleet" of F-45s and FP-45s) and just collect them, as I love the looks of both units, but I'm in a budget cruntch myself and I'm resorting to drastic measures, even if theyre not easy.

   I found a person at a recent train show who sells a lot of undecorated Athern locomotives (a lot of F-45s) and I figure, I could buy all that I can, and do what I would like with them (My first plan is to model every FP-45 the Milwaukee Road owned) However, with the budget cruntch i'm in, as mentioned, I have to cut back, and the person has the cheepest undecorated locomotives I have found (the F-45 I now have  was priced at $17.00, however, when I asked, he sold it to me for $10.00) and I will like to get more from him. Unfortunately, he mainly had SDT40-2s, F-45s and a few U-boats mixed in. So, I would like to try and undertake the challenge of modifying some of the F-45s into the FP-45s.

I think it would be a good learning experiance.

If you look,

I think you could find an old Athearn Fp45 for about the same price or less.  Otherwise, you will need to add about 4 scale feet to the length of both the body and the frame of the Athearn F45.  Then of course you need longer drive shafts from the motor to the rear axle, and you haven't even started adding details.  Athearn was selling both the F45 and the Fp45 when I fist came back into the hobby in 1980 or so.  There are a lot of them out there, they weren't doing limited runs back then either.

Frame length research

Some notes I have lead me to believe that the FP-45 frame and the tunnel motors' frame is the same.  The SD45T-2 was, at least originally, on the same frame  as the SD-40T-2, with just a longer long hood, and shorter snoot end, to accommodate the V20 engine.  I compared a photo of an FP-45 with an SD40T-2, and they appear the same.  But, by all means, research this.  I'm just trying to point you in a direction that allows you to do a loco kitbash without having to invent a new frame and drive train.  Maybe an SDP40 or 45, if available.

edit:  SD40T-2 length overall is 70'-73/4"

I'll try to find an FP-45 or an SD40T-2 somewhere

 Alright, it seems that re-inventing something alredy made may not be sutch a good idea. I'll try to find an alternitive.

Another question,  On a railpictures.net, I searched for Milwaukee Road locomotives for fun, and I stumbled across a locomotive called the F40C (in the photo I found, MILW 41) It looks like an F45, but some of the photo descriptions say it's based off of the SD 40-2. Can anyone tell me more about the locomotive?

Wikipedia to the rescue....

http://en.wikipedia.org/wiki/EMD_F40C

The EMD F40C is a 6-axle diesel locomotive built by General Motors Electro-Motive Division between March and May 1974. This locomotive was built specifically for commuter service use. It is powered by a 16- cylinder EMD 645E3B, which can produce 3,200 horsepower (2,390 kW). It shares the same common frame as the EMD SD40-2, giving it an overall length of 68 feet 10 inches (20.98 m). All 15 examples were originally built for purchase through a publicly funded agency to provide for upgrading of service over Milwaukee Road lines west and north from Chicago to Elgin and Fox Lake Illinois.

(there is a lot more at the link)