I've even considered mounting the brass bodies on modern frames. You don't necessarily need an A - the brass versions have a nice, un-swinging pilot - which is appealing.
But, all that is just spitballing - I have never even measured any of this. And, the proportions of the nose, etc., as mentioned above, are not truly sharky. I just like the looks of brass diesels and fixed pilots - it's not a switcher, necessarily.
And - are you sure that yours are actually "bad runners", per your needs? They may go down the track better than you fear. Flywheels are preferable, but not necessary.
Modern Williams scale Alco FA's have a fair version of the AAR side frames you need, and I think that they are a bit under-scale-length. But, I've never measured.