first of all note down the address(es) with balance
then you can go to this website https://iancoleman.io/bip39/  and save the page locally (after that disconnect internet access)
open your original electrum wallet file with notepad and copy the xpub from keystore.
Open the saved page on your computer and enter your xpub value in the BIP32 Root Key field.
After that click on BIP32 tab and see if your address is in the list.
From here you can generate more addresses on the same derivation path or change the derivation path until you find the address you are looking for.
After you find the correct settings you can replace the xpub value in the BIP32 Root Key field with the xprv key from electrum wallet file and it will generate the coresponding private keys which you can import in your preferred wallet (bitcoin core / electrum)