This legacy algorithm can be enabled using Sftp
object's Settings.SshParameters.KeyExchangeAlgorithms
property:
var client = new Sftp();
client.Settings.SshParameters.KeyExchangeAlgorithms |= SshKeyExchangeAlgorithm.DiffieHellmanGroup1SHA1;
client.Connect(serverName);
...
For additional information, see SSH ciphers and Setting connection options.
Also, please be aware that the diffie-hellman-group1-sha1
cipher is not secure.