In case somebody needs a good battery replacement, I've purchased one from this company in 2019 and I am still happy with its performance (2022): Wannsee-electronic Galaxy S2 i9100 battery

^ +1 for polarcell! Got a N900 battery just a year ago, better capacity than the original.

PS: how stable/usable would you say your build is, over two years after your initial statement?

I am still happy with the build. It is stable and usable, however, I am not using the device very often now. (Sfos is a bit slow on it and got some other devices to work with and test.) I still build sfos to the S2 but some things do not properly work. I've planned to make waydroid work on it but that would require a lot of work as the port uses a cm12.1 base and the device has a very old kernel (version 3.0). I've started to re-base to los17.1 but unfortunately, that also lacks some stuff that is required for Waydroid. Therefore I've put it aside and probably will come back to it later.

The 4.4.0.64 build is available to download. Link's in the main post.
From now on the kernel is installed when flashing the zip file.

On the first page, I've removed some old kernel/sfos release links because the post length is limited to 1500 characters.

Some useful tips:

• Low power mode aka ambient mode (in Fingerterm):
  
  Code:

  mcetool --set-low-power-mode=enabled

• Disable lock screen animation (in Fingerterm):
  
  Code:

  mcetool --set-lockscreen-animation=disabled

Oh, nice.
Do you have more useful tips, or the source of these useful tips?
I've been looking for ways to reduce animations.
What does the other one (low power mode) do exactly?
I'm trying both anyhow... do they need to be reapplied each boot?

What does the low-power mode enabled do?
This is meant to be to show the time&date&weather when the device is pulled out the pocket without waking up the device with the power button.
You can try it following these steps:
- lock the screen with the power button
- cover the proximity sensor at the top for a few seconds
- uncover the sensor
- The clock should appear in blue for a short time.
(I am not sure how reliably this works on newer Sailfish versions. It worked well on older releases but Jolla might change this because on some Sony models there is a problem with the proximity sensor when the device is in a deep-sleep and they couldn't fix that. Then they decided to switch this off when a device goes to deep-sleep. What I am not sure is whether they did this for all devices or just for Sony devices.)
No, these don't need to reapplied after reboots.

Thanks. It seems to do nothing on my Sony Xperia XA2, so I disabled it again.
The disable-animation thing does exactly what it advertises.

Yeah, on the Sony, it only works right after you lock the screen with the power button. Although, that is useless as it is meant to be working when the device is in deep-sleep and you pull it out from your pocket. I loved this function on older Sailfish versions, but sadly it is dropped from newer releases because of the issue on Sony devices.

The 4.4.0.68 build is available to download. Link's in the main post.
(No need to flash kernel separately.)

The 4.4.0.72 build is available to download. Link's in the main post.
(No need to flash kernel separately.)