What is the most recent pushpool version and where to get it? I found already the following pushpool versions:

https://github.com/jgarzik/pushpool
https://github.com/ArtForz/pushpool
https://github.com/alexhz/pushpool
https://github.com/CryptoManiac/pushpool


(can't post more than 2 links with my current reputation, so made a block)

All of them seem very old already.

I made a research on how to modify the difficulty in pushpool's server.json and found examples on how to set the following options rpc.target.bits and work.expire, but NONE of them seem to work for the mentioned versions.

So what's the newest pushpool version and where can I download it?

EDIT: Tried as described in the following post, but not working. Difficulty doesn't change and nothing is written into shares. MMCFE Not Reporting Stats from pushpool/wallet
Since you tagged your question with litecoin, I believe that you are looking to serve a litecoin pool. For my needs https://github.com/ArtForz/pushpool
did the trick. It added scrypt support and allows you to rewrite the target difficulty (just remove "rpc.target.rewrite": true and replace it with "rpc.target.bits": 18 in the config).