Hi Andrew,
You might want to sit down. This could take a while.
I can think off the top of my head of maybe five sources for O scale diesel detail parts. I'll list them out and try to include some of the pros and cons to each as I see them.
1) Des Plaines Hobbies
www.desplaineshobbies.com/store/They have lots of O scale detail parts. The problem is that their website is not particularly useful in identifying what they are selling (usually no photos of detail parts). I would suggest looking through the list of parts and noting the things you think sound like what you want. Then I would give them a call and ask about your list. If you ever make it to the states for the Chicago March Meet (regarded as one of about 3 or 4 best 2-rail shows), then a side trip to Des Plaines Hobbies is a must.
2) Precision Scale
psc1.virtualfocus.com/Similar to Des Plaines in that they have a lot of parts that are very difficult to access. They do not have any online catalog, but will be happy to sell you a paper version for $8.00. Don't know about shipping to Australia. If you can find a hobby shop that has a desk copy you can find things in there. For instance Caboose Hobbies has desk copies, but they are out of date. Better than nothing I suppose.
3) Atlas O
www.atlaso.com/welcome.htmAtlas has a modern online store. Most detail parts don't include pictures, but parts diagrams are available for many of their locomotives. Frequently, the radiator screens (I presume this is what you mean by "long air intakes") are available as separate pieces for many hood units. I'm going to use some parts from the sd40 and gp60 to scratch/bash an sd40-2 and a gp38-2. In particular the dash 2 cab, sub base, and nose from the gp60 and engine compartment hood from the sd40.
Parts diagrams can be accessed by clicking on "New Products" on the Atlas O homepage, and then on "Parts Diagrams" on the right side of the new product page.
www.atlaso.com/parts.htmIf you can get your hands on an Atlas Mp15dc you could use the drive components to repower your gp38-2. The Atlas switchers are considered to be some of the best running O locos, mostly by virtue of their single motor drive, enclosed gear boxes, and larger flywheels. If you have the ability to build or modify a frame to accept these parts, all you would need are lengthened drive shafts to fit a gp38-2. You can not use the gp blomberg trucks from the gp60 or gp35 or f units for this as they are designed with a motor and smaller flywheel mounted to each truck.
4) P&D Hobbies
pdhobbyshop.com/show_subsubcategories.php?category=Trains&sub_category=%26quot%3BO%26quot%3B%20Scale%202%20RailSimilar to Des Plaines Hobbies. They have various detail parts in stock with a sort of effective online storefront. They also have some gorgeous brass cast blomblerg trucks. A bit pricey, but wonderful to look at.
5) Bill Davis Brass
No traditional website to speak of. Sells on ebay under the name "brassmonger". Email as I know it is bdavis148@aol.com . The reason I mention him is I believe he bought the parts stock from Overland when they got out of that part of the hobby business. People that know him say he's a good guy and contacting him through the email address would be the best bet. He is usually at the Chicago March Meet.
You're going to have to hunt and peck to find what you need. In some cases scratch building certain parts will be easier or absolutely necessary. You are not going to find anything like Cannon & Co. or Details West or Detail Associates in O scale. One thing you might consider is getting the Cannon & Co. HO equivalent, scaling it, and then scratching it in O scale. Their parts are considered to be highly accurate and they have a comprehensive line of parts for EMD second generation locos.
For ideas about some of the lighting, sound, and control issues you mention, check out this website for the Appalachian and Ohio layout here in the Denver area.
www.aorailroad.com/phpBB3/viewforum.php?f=3 It's a great layout, and a bunch of good modelers. Many of the posts by Bob (Sobel) in the discussion forum involve tweaks and implementations for sound and lighting. Many other posts discuss scratching and bashing of various O scale locos. None that I remember are of the gp38-2, but some of the work could inspire you. In particular try the following forum post for ideas about lighting.
www.aorailroad.com/phpBB3/viewtopic.php?f=3&t=121I hope this gives you some ideas to start with. While I haven't really done much modeling, I certainly do a lot of
planning of modeling. Hoipefully some of these sources will provide you with information you can use.
Jim