As much as I wish I could have a single answer against the SMG and BDG, I do not think it is much good, just a worse version of the ..d5 SMG declined lines.
Also, the line itself is actually very old:
I already see something wrong with the line played. That doesn't exactly rule out the opening though.
I would change white's play in the above diagram starting with 6.c3, giving the king a nice spot on c2 if needed and preparing to hold his extra pawn with b4.
Either way. Qxc5 in the diagram you gave was the mistake. It gave White the extra tempos. Qxd1 is a much better alternative.
I disagree, white still has an extra pawn in the Qxd1 variation, and I do not think blacks compensation is enough. The move that was really a mistake by black was 5.Qe5?!, 5..Qb4+ was much better.
All that said, keep in mind that the point wasn't that the game demonstrated best play, but rather that the variation has been known for a long time, and despite this very few strong players have used it.
I have taken some time to try to find a better way to fight the Smith-Morra gambit. I think I may have found something interesting that I'd love to share with everyone. I like to announce as a nice Countergambit for the Smith Morra.
Hope you enjoy!