There is really no difference between 50n and 47n. The box film caps are normally 5% or 10% tolerance. So a 50n cap with a low value but still with in 5% is 47.5n. And if the 47n has a high value it could be 49.3n. If the caps are 10% then the overlap is even greater. And if you calculate the cut freq's 50n is 14.47Hz and 47n is 15.4. That is below the range of human hearing and 1Hz is nothing.
The pedal this is based on, is from the early 70's. Back then a 50n cap was a lot more common, now its a 47n.
Just curious, what transistors are you using? Both the ones called for a a bit rare.