Hey guys, sorry for the late update! My M20 battery change went disastrous. The battery cell fits in the F1 battery department. The cell is just a tad narrower, so no problem with that.
It turned out that the M20 battery has an additional temperature sensor (LC77AY-1) soldered between the cell and the circuit board. It needs to be remover, so you can have the contacts in the correct spot.
However, during soldering process, I managed to short-circuit the cell through the charging board (one contact was soldered, and the other contact touched the charging board output connector and melted it halfway).
I still finished the job with lowered expectations. Connector was still usable and charging was working. I decided to test the cell with an opened phone and during initial charging the cell swelled a bit. It took very low charge (something around 1000mAh). I already knew this cell was ruined. I don't know if the short circuit or the fast charging is what killed it. I then removed the M20 cell and re-assembled my old battery.
I looked again at BM4E clones at Ebay and ordered one with plenty of feedback. Battery came few days ago and turned out to be of good capacity. Accubattery measured 3987mAh charge from 1 to 100%. I'm very pleased with this new battery, so I will share the link to the Ebay listing here:
Find many great new & used options and get the best deals for Xiaomi Pocophone Poco F1 BM4E 4000mAh Battery M1805E10A at the best online prices at eBay! Free delivery for many products.
Price is GBP £13.95
Unfortunately this seller only ships to the UK.
Last 2 pics are my new battery.