Replace Attic Fan Without Going on the Roof
When I've replaced motors in the past I replace with the exact part if you went with that approach you shouldn't have to worry about clearances but it would be painful to work within that constrained space.  

It doesn't look like this fan is designed for serviceability from the inside but there are a lot of things like that so it doesn't surprise me that you can only remove from the roof.

A couple other thoughts.  

It looks like you could pull the whole fan into the attic (minus the exterior wall cap portion).  Depending on how the exterior portion is attached you might be able to somehow secure the exterior portion from falling out and then detach the whole fan assembly from the inside and pull it through.  This might save you a trip onto the roof but you'd still probably need one trip to resecure the wall cap portion.

You could also cut the vertical supports 1/2 way and then scab a new piece of metal that is drilled and bolted to the original overlapping the cut.  This would give you future serviceability.  

I am not sure what approach I'd take.  Depends on if you plan to keep the house, your kids will inherit it, how hard the roof access is, your siding details, cost of a full new unit, availability of parts for the current unit.  This won't be the last time the fan motor dies.