Verified answer

The ideal efficiency "eta" is given in terms of the ABSOLUTE temperatures (Kelvin!) of the hot and old reservoir between which the engine operates:

eta = 1 - Tlow/Thigh

From the data given we can first determine Tlow (the absolute temperature of the cold reservoir):

Tlow = Thigh*(1-eta) = 732K * (1-0.30) = 512.4K

Now that we know the temperature of the cold reservoir, we can fix that and ask what the temperature of the hot reservoir should be to increase eta:

Thigh = Tlow / (1-eta) = 512.4K / ( 1 - 0.43) = 899 K

That equals 626 Celsius.