Without looking at the schematic, I have a hunch the key phrase in the original question was: "...to the source of Q2 in the VFO section"
If the condenser is there only as a DC block the value isn't critical. But, as you're aware, larger condensers will pass lower frequencies more readily and 5 mmf is a very, very small amount of capacitance. Which leads me to suspect the 5 mmf value was chosen to act like a high pass filter in a circuit relying on the overtone of an oscillator.
I wouldn't stray much more than 50% and that takes you all the way up to 7.5 mmf. Which isn't very far, and if Heath
kit could have subbed a more common value (like 68 mmf) I suspect they would have.......