I created an address at high index(about 50,000) by script and sent some bitcoin to the address.
What was that script and why can't you use that again?
Also why aren't you using another script to produce the keys? I'm assuming you know how to write basic code so:
1. Download a list of all bitcoin addresses with a balance from here:
https://bitcointalk.org/index.php?topic=5254914.02. Write a script to derive child keys from your electrum mnemonic
3. Create as many addresses at as many indexes as you like and check them against the list in step 1.
4. Create a new Electrum wallet using WIF (private key) and import the key at that index to it to gain access to your funds