Right : 51.Nh6+ Qg7 (51...Kxh6 52.g4#) 52.Rxg7+ Kh8 53.Rg8+ Kh7 54.Rh8#

This opening can be very hard to hold for whites!
But there's no comparison with the Alekhine variation of this opening:

1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.e4 (in place of Nf3)Nxe5 5.f4 Ng3; and now you have to be a master to hold the position coz it's gonna be very sharp.

After 34.Bf7, instead of 34...f5, perhaps 34...c4 35.Bxh5 c3 36.bxc3 Qxc3 is best for Black and perhaps winning.

36...Qg3 doesn't seem to help. Better seems 36...Qf4. So if 37.Nf7+ Kg8 38.Nxd8, then 38...Qxh4 and 39...Qxe1.

After 36...Qg3, Capa played 37.Nf3. Why not 37.Nf7+, and if 37...Kg8 38.Re3 Qxh4+ 39.Rh3 and 40.Nxd8 should win.

Now Black wastes a tempo with 37...Qd6. Better seems 37...g6, trapping the bishop.

After 38.Ne5, White threatens a family fork with 39.Nf7+. Black played 38...Qf6, but any king moves looks better. Black is still OK if White tries 39.Nf7+? Kg8.

39...Rd2 looks weak. Perhaps 39...g6 and if 40.Nxg6+, then 40...Kh7.

Capa played 40.Bxc5, but 40.Nf7+ looks stronger after 40...Kh7 (40...Kg8?? 41.Re8+ Kh7 42.Rh8 mate) 41.Ng5+ Kh6 42.Be8 (42.Re6? Kxh5 43.Rxf6 gxf6), threatening 43.Re6.

40...Rc2? looks like the losing move (40...Rxb2 would be worse). Black should still try 40...g6 and 41...Kg7. Now, after 40.Bd4! White has all kinds of threats on the queen and king.