I don't think any commercial malt will give you the combination of flavor, color and diastatic power you want. You'd want a malt with a color around 10L, which would be something like a Dark Munich. A SMASH from that would probably be overly sweet and insufficiently roasty for a porter. You could still SMASH and add enzymes (available and cheap at your LHBS), use brown malt and about 20-25% malted wheat, or make your own: http://perfectpint.blogspot.com/2012/05/making-diastatic-brown-malt-again.html
That last option would seem to require the sort of total commitment I both admire and question. As he pointed out on that link, even if these sorts of historical re-creations are 100% successful, you'd never know it, since you lack an exact frame of reference.