Grab the schematic and follow along.
The new noise source is used as the signal input to the Sample & Hold circuitry. U4a is a buffer, Q2 does the sampling, and C7 does the holding. Use any decent N-channel FET for Q2. The Pro-One's LFO square wave is used as the clock signal for the Sample & Hold. The 555 timer is used to turn the LFO signal into a short sampling pulse for Q2. The Sample & Hold output signal goes to the LFO Modulation Source switch.
Note that if you make the modification to allow changing LFO pulse width, it is possible to set the LFO pulse width too narrow for the Sample & Hold to work correctly. If the Sample & Hold doesn't seem to be working, check that the LFO Pulse Width pot is set to the middle position.