Recovering an old Multibit wallet file (76 characters long)
Does anyone know the correct method of decrypting this?

It doesn't seem to be encrypted. The comments at the start of the file don't mention any form of encryption. It only mentions a common form of encoding - Base58. Encoding is not encryption so no decryption is needed.


#   The Base58 encoded private keys are the same format as
#   produced by the Satoshi client/ sipa dumpprivkey utility.


That Base58 key ought to be able to be imported into several different modern wallet programs.
See

Private keys importing in format got by dumpprivkey
How to get the private key in hex from the output of dumpprivkey
https://btcinformation.org/en/developer-reference#dumpprivkey