Jim,
When you write of "gear" do mean the whole assembly? Taking gears on and off which are usually peened or swedged on in some fashion with metal hardened is not a job most fool with.

Now the ford gear you have (and I assume you really mean the whole bendix drive) was too small. so I go with ,too small to fit the armature shaft, not too small to fit gear back on the bendix drive unit it self.

I bet you have a Ford Model T bendix unit which has a small armature shaft starter. Also the earliest Model A starters did too...

The most commonly found bendix drive unit for the MODEL A Ford though has a larger I.D. bendix drive unit....It is a perfect fit on the extra Durant Star starters I have in my pile.

Whether the D40 has the same size starter shaft as my Early Star starters I don't know? But the Ford(and also on my Star) size was very common and about as big as they get in armature size diameter of the time.
The number of teeth being less or more may or may not be an issue with the ring gear mesh,,but can have a speed and torque effect that may or may not be a big deal.....
Length of assembly (Ford A to My Star )is very close if not the same and if needed may be able to adjusted a bit with a shim or grinding.

Where Are You From? Ct

Do You own a car built by Durant? '23 Star Touring