Great pix jere! Much better idea of what you are talking about here. Now this is just me, and I'm not trying to be prescriptive here, I'd be all over that old motor base plate to build an adapter on it. But that's just because I see the old motor as a liability, sorry. And with the thought of it out of the way modding that base plate would probably mean making a motor mount out of the dc motor mount(basically like Jonathan suggested, but using the old parts) and as long as it's centered and square with the old plate then it's only the depth and having enough shaft of the dc motor sticking out. Bad thing is you're going to have come up with a seal on the motor shaft because of the gears running in an oil bath.
Then I would drill some good sized holes on the bottom of the main case for air flow and on the bottom to help with the filtering. And take the old end casting and where the embossed(?) parts are, take them out for the other air flow. Drill, cut, whatever, and put screen or whatever over them. Then you can bolt the old motor casing and end back on.
In my studies on dc it's supposedly a pretty flat power curve so when you run them slow they don't get as weak as an a/c. The only bad thing is the lowered air flow. I wish I'd known about treadmill motors when I did my redo on my bandsaw because every once in a blue moon I do some wood and miss the high speed. With this set up no changing pulleys no muss no fuss potentially.
Just my deflated 2c.