I have an old multibit.key file that I'm trying to import into Electrum.
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Alternatively, is there a way to unencrypt it without using Multibit Classic?
The guide that I wrote earlier as linked by ranochigo above is your best bet
without Multibit Classic.
I will just add that unless you know what password was used to encrypt that .key file (and/or have a
very good idea of the length of the password and whether it was UPPER/lower/numbers/symbols etc) then your chances of decrypting that .key are virtually zero.