Perhaps the back half of the case can be switched between the 10DN and the 10SI. I've never done it, but they are close to the same from my memory. There are threaded mounting bosses in the back of the case for the regulator in the 10SI, which I doubt the housing of the DN would have.
Or, you could grab a 10SI from the yard and just buy a rebuild kit to change the brushes, regulator, etc. Should be cheap as it's the most common alternator out there.
Greg