Migrating legacy IPSEC tunnel to connections, can't add proposal

[jonm]

I'm trying to migrate a legacy site-to-site IPSEC VPN tunnel to the new Connections, but the proposal used isn't listed in the dropdown.
How do I get around this?
It's aes192-sha1-modp1024 and I can't change it because of the router hardware at the remote end of the connection.