I dont know about finding English sources since even most references in Poland are anecdotal at best (Im sure there are academic/scientific papers, just not on the net/google). Kompot is aka "Polish Heroin", and it's not a moot term as the acetylation it implies is real...
...and here goes the magic: you get a bunch of strongly acidic (h2so4) kationit, basically a non-soluble acid in solid form that turns the basic alkaloids from the PPT into salts sucking the bittery goodies up in the process and actually turning the pod soup sweet while leaving all the organic matter behind, which is promptly discarded. Then the kationit is washed anew and the solution of morphine sulfate is cooked (optional:a.charcoal) and basified with ammonia water forming freebase which crystalizes upon evaporation. This morphine freebase "plaster" is dissolved in ether/acetone and refluxed for 4-5 h [@85 C optimum], during which acetic anhydride is added at 4x the suspected content of morphine once the solvents have almost evaped, which yields a decent percentage of heroin in the final product, alongside monoacetylmorphine/unreacted morphine and trace opiates.
The above procedure is the classic recipe, you can experiment with different acids yielding different salts, additional filtration, chloroform extraction, etc. for even higher purity/yields. Im sure some hardcore fiends opted for dirty kompot which is basically a sieved, HCL'ed and acetylated version of PPT, full of all the opiate hydrochlorides, but still a respectable portion of morphine and its acetylated derivatives (and some impurities which got past the acid extraction).
Edit: Solid kationit is used as it's even more convenient than straight up acids, especially considering its availability, right with the water softeners at your local fish hobbyist shop. Same mechanism of sucking stuff outta the water (trading ions).