I've recently been exploring some of Electro-harmonix early pedals... The LPB-1, the Screaming Bird & the Mole. They are really simple circuits & fall into the family of Boosters - all using a transistor for amplification.
R1 & R2 can be 430K & 43K respectively.
The transistor is a 2N5088 on my vintage example but you could also use a 2N5133 or BC239
if building your own.
The LPB-1 and the Screaming bird pretty much are the same circuit apart from
two capacitors (C1 & C2). The first part of the circuit (C1 & R2) work like a high pass filter.
Increasing the value of C1 should let in more bass. If you decrease it, more treble should appear.
With 0.1uF, most of the input signal goes through. In the case of the screaming bird, EHX have indeed reduced C1 to introduce more treble.
The Mole has a slightly different design even though one can see a family resemblance to the LPB-1.
R2 & C3 form a passive Low Pass RC (Resistor-Cap) filter .
+ Electro harmonix time line
+ My LPB-1