Unmodified they have a gain of about 20dB and NF around 4dB. I replaced all the FETs with MGF4919s on one and CE3512K2-C1 on another. I also converted the 1st stage to single ended. That reduced the NF to <1.4dB and increased the gain to 20-25dB. It also reduced the centre frequency. I didn't bother tweak it after that.
The bias circuitry is quite fragile, physically and electrically. If you mod. one, be careful you don't damage it. It would be better to replace it with an alternative e.g. one of the Zetex bias controllers.