You would already know that the marker readout gives you a rough idea of the frequency but if you are trying to tune something then the sweep rate with a minimum RBW of 1kHz for the best accuracy is not very fast or practical.
This instrument, due to hardware limitations, could not provide a full range frequency indication. Conceptually however, considering the VNA design has a software defined second IF, it could be modified to give a frequency readout or indication within a selected frequency range of 70MHz, if you knew basically what part of the band you wanted to be in. The 70MHz maximum range limit is determined from the maximum number of display points multiplied by the maximum RBW.) This gets you a swept range with no holes for a primary sweep, then you could use the marker to close in to the actual transmission using lower span and RBW settings before moving to a readout mode when you are close to the required setting.
USING THE CURRENT KC901V DESIGN FUNCTIONS TO ADJUST FREQUENCY
If you have a desired frequency you wish to tune a variable frequency transmitter to, then follow this process.
1/ Go to Spectrum analysis mode.
2/ Set the desired frequency
3/ Set the RBW to 1kHz (the minimum)
4/ Select span mode then zero span.
5/ Tune your device for a peak amplitude level on the display.
Though this process is not greatly accurate because of the fact that the RBW is still quite wide at 1kHz, you can note the high and low tuning adjustment points at which the level drops by say 3dB and then use the centre point between these.