no matching key exchange method found. Their offer: diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 user@legacyhost
or in the ~/.ssh/config file:
Host somehost.example.org
KexAlgorithms +diffie-hellman-group1-sha1
More info on OpenSSH