I have been trying to make the Hamppe-Allgaier make work lately, but I don't trust it. This is an excerpt.
1.e4 e5 2.f4 exf4 3.Nf3 g5 4.Nc3 Nc6
1.e4 e5 2.Nc3 Nc6 3.f4 exf4 4.Nf3 g5
5.h4 g4 6.Ng5 h6 (I have not looked at d5 yet) 7.Nxf7 Kxf7 8.d4 f3!
Another interesting option is the transposition with 8...Nf6 9.Bxf4 Bb4 10.Be2 and only now d5.
9.Bc4+
9.gxf3 Bb4 10.Bc4+ (10.gxf4 d5) d5 (Ke8!?) 11.Bxd5+ Kg7 just transposes to the main line.
Estrin/Glaskov's recommendation 9.Be3 is also met by Be7; sooner or later White will have to play gxf3 anyway. After say 10.Bc4+ Ke8 11.gxf3 Bh4+ (Black can do without d5) 12.Kd2 d6 White has nothing for the sacrificed knight.
9...d5 10.Bxd5+ Kg7 11.gxf3 Bb4 12.Be3 Nf6 13.Bc4
13.Bxc6 bxc6 14.Qd3 gxf3 15.o-o-o Rf8 16.Bf4 f2 17.Rhf1 Ng4 18.Qg3 Kh7 -+.
13...Qe7
13...gxf3 now looks playable too: 14.Qxf3 Nxd4 15.Qf2 Nc6 and square e5 lures the knight.
13...Qd6 14.Rg1 Qh2 15.e5 Re8 16.exf6+ Kxf6 17.Qe2 is dead equal, Castelli-Gasloli, Buenos Aires 1967.
14.Qe2
Gallagher-Hresc, Geneve 1991. Here 14...Rd8! looks good, as 15.o-o-o Bxc3 16.bxc3 Qa3+ 17.Kb1 Qxc3 gives Black a huge advantage. But 15.Rd1 gxf3 16.Qxf3 Nxe4! 17.Rg1+ Kh8 18.Qf4 Bf5 also leaves White without compensation.
Initially I also thought 8.Bc4+ an improvement. After d5 9.Bxd5+ Kg7 10.d4 Black has the extra option 10...Qf6 11.e5 Nxe5 12.dxe5 Qxe5+ Zhelnin-Tiulin, Cherepovets 1973. I think Black is better.
Black also can try 9.Bxd5+ Ke8 10.d4 Bg7 11.Bxf4 Nxd4.
Finally Blackburne has played 8.Bc4+ d5 9.Nxd5 a couple of times. Here I have found something strong for Black: 9...f3!
a)10.Nxc7+ Kg7 11.Nxa8 fxg2 12.Rg1 Qxh4+ 13.Ke2 g3 -+.
b)10.gxf3 Ne5 11.Be2 g3 (c6 looks good too) 12.d4 g2 13.Rg1 Qxh4+ 14.Kd2 c6 15.Ne3 Bh3 -+. Not better is 11.fxg4 Nxc4 12.Qf3+ Ke8 13.Qc3 Qd6 14.Qxh8 Qg3+ 15.Kf1 Nxd2+!! 16.Bxd2 Qf3+ 17.Kg1 Bc5+ 18.Be3 Bxe3+ 19.Nxe3 Qxe3+ -+.
An amazing change of roles.