At move 21 white clearly has the better position.

Actually at move 21, it is Black that has the better position.

Lowenthal's move 20.Qb7, gave away all of his advantage and permitted Morphy to gain the advantage if he had replied 21...cxd4!.

Fritz 9 provides the following evaluations and variations:

After 21.Qc6 cxd4 (-.46) (19 ply) 22.Bg3 Ne7 23.Qc1 d3.

Now an updated evaluation shows additional advantage for Black (-.78) (18 ply) with a suggested continuation of 24.a4 g4 25.Nd2 Ng6.

At this point Fritz's evaluation is at (-1.19) (18 ply).

Another variation is: 21.Qc6 cxd4 (-.52) (19 ply) 22.Ra1d1 Nxe5 23.Rxe5 Rd7.

Now an updated evaluation again shows additional advantage for Black (-.83) (19 ply) with a suggested continuation of 24.a4 bxa4 25.Bxa4 d3.

At this point Fritz's evaluation is at (-1.32) (18 ply).

Both of these lines are considerably in favor of Black. Additional work would be needed to determine if White can hold the draw in either variation.