To mount the Athearn trucks, you'll need to drastically re-work the floor/frame of the brass diesel. A better choice might be to use the entire Athearn chassis, if you can modify it to fit. In addition to the Athearn trucks not being compatible with the brass floor, the driveshaft couplings would need to be modified, due to a mismatch of shaft sizes with the brass model's motor.
What brass diesel are you planning on buying? While you can make any of the modifications that I've mentioned, and probably a lot more (I've remotored, re-geared, and rebuilt all kinds of brass locos, both steam and diesel, so I'm not just slappin' my gums together, here) :roll: :grin:, but unless you are really interested in doing this work and/or spending the money for the parts, and possibly, the tools to do this work, I'd advise you to think long and hard about it. As I said, unless the brass model is of a unique prototype, chances are that you can get a better looking, better running, and more accurate model by buying plastic, and it'll probably cost you less, too!
When it comes to brass steamers, this is a completely different subject. Most brass steamers are road-specific, and the better ones are class-specific, meaning that your chances of getting a ready-to-run plastic version are much slimmer. This in turn makes it much more worth your trouble to upgrade such a model.
I certainly don't wish to discourage you from this pursuit, as this type of work would be a dead art if it weren't for those of us who still practise it. There is a great deal of enjoyment and satisfaction in doing this type of upgrade, but you need to be aware of the pitfalls, too.
Wayne