modifying a few lines of code in the iancoleman's page would make it work for electrum seed you just need to replace a word of "mnemonic" to "electrum", and then disable its default checksum function
One of the key differences is that the BIP39 spec (and by extension IanColeman tool) uses a passphrase of "mnemonic" + yourpassphrase... whereas Electrum uses a passphrase of "electrum" + yourpassphrase.