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I had encrypted 2 MySQL databases with AES_ENCRYPT in ECB 128 bit mode. Now I forgot my encryption key, but I have plain text (I have one database without encryption and same with encryption). How to find the encryption key so I can decrypt my other database?

I heard about known plain text attack, how to do this? Is there any tool in Kali Linux or some python script on GitHub?

Realistically, no you cannot. AES is very resistant to known plaint text attacks like most block ciphers.
It's lucky you didn't lose any information and have the original database backup because your only real option would be to try brute force the encryption, which is likely to take longer than the length of the universe (unless you can greatly narrow down possible keys by almost remembering your password?)

What you are asking for is a known-plain-text attack (KPA). The weakness you describe in ECB is where two encrypted blocks with the same plaintext are encrypted with the same ciphertext.

This just means you know that two encrypted blocks in the ciphertext look the same, it doesn't mean that you can recover the plain text easily. The answer is: no you will not be able to recover the key.

In theory it is possible, but in practice no. but you can try to guess it. Given a plain text and a ciphertext, you can try every possible key to see which one works. For AES-128, 340 undecillion keys.

As shown above, even with a supercomputer, it would take 1 billion billion years to crack the 128-bit AES key using brute force attack. This is more than the age of the universe (13.75 billion years).

At the time you created your key or password, you probably stored it in a file or wrote it on a piece of paper. Get yourself an "undeletion" program (good ones are available for free, just search) and look over your previously deleted files... You may find it there.

This is called a known plaintext attack against a cipher and is a major design consideration. What you want to do is considered to not be possible barring any major revelation into a weakness in AES. There is another Q/A here that might help you understand further:

https://crypto.stackexchange.com/questions/1512/why-is-aes-resistant-to-known-plaintext-attacks

Ciphers only operate on a fixed length of bits, so chaining modes are used to encrypt arbitrary lengths of data. ECB is the most simple one, it just encrypts one block at a time. The main weakness is that identical blocks produce the same encrypted output which is a major leak of information. It also creates opportunities for replay attacks where an attacker can just re-send encrypted blocks that they think they know the contents of. Unfortunately this does not get you anywhere closer to key recovery with a known plain text.

One viable attack for you might be if you used a password to generate the key, and you remember for example that it is exactly 12 characters long and has three digits. This majorly limits the password space and might allow a brute force analysis to work.