Do NOT power-vent that room into living space. You never want to vent a room containing combustion devices into living spaces...as you state, that's asking for CO problems. It's not an issue of the furnace being defective, it's an issue of you sucking exhaust out of the normal path (the chimney) due to negative pressure.
You could get around this by using a direct vent/intake system where it becomes a completely closed circuit (furnace draws directly from outside).
I'm not an HVAC specialist, but a 90 degree room seems to me that there's something else amiss here...like maybe your circulating fan is underpowered?
First, realize that with a single zone system and multiple floors, it's difficult to get the temperature correct everywhere. Also, since you mentioned this was an attic space, you're likely against the roof and getting heat from every wall plus from the computers and people in the space. In short, you're fighting a losing battle.
I've got a similar challenge, though not to your extreme, and here's what I have done. First, learn how to adjust the baffles on the ducts. In the summer, you want as much as possible going to the high floors, and the reverse in the winter. Get someone to feel the air coming out while you're pushing the baffle all the way to one side or another. For me, it made little difference until I pushed it all the way.
Next, exclude any rooms you don't need to heat/cool, e.g. a guest room or storage space in the basement. Shut the door and close the vents to reduce the effort you're putting on the HVAC.
Finally, I swapped out my return grille with models that accept a filter (they are hinged and open after releasing two small clips). These return grilles are special orders from the big box stores, but they carry them. I still leave a thin blue "rock catcher" filter directly on my HVAC blower itself. And I put a filter in every return except the upstairs in the summer and downstairs in the winter. The idea being to suck out as much air as possible from location that needs it.
This means that switching between heat and cool is a process, flipping the baffles and swapping in and out filters. But a little effort is better than a lot of sweating.
One last suggestion, since it sounds like you're running a small server farm in your attic. Either move that down to the basement, or consider replacing the window unit with a ductless mini-split system. You get to see out the window, and these systems have configurable thermostats.
Best Answer
Definitely don't use WOOD as suggested by DMoore! Yes, consult your local Ohio codes.
A new material, probably not rated or considered in Ohio code, but used extensively outside the US, is MGO board, which is fireproof, waterproof, mold proof... I wish this would get certified, as it's a much better idea than drywall in the first place! This is NOT Chinese drywall, which is just poor quality unregulated imported garbage. A reputable MGO board company will probably have certifications soon for this type of installation.
At least be glad that the old asbestos board, which was used extensively decades ago, is gone, after the medical concerns surrounding that material arose.