The HC5345 is a SeaStar cylinder, not a Hynautics. I'm not sure you can use a SeaStar cylinder with a Hynautics helm/pressure setup...
I'd ditch the old Hynautic and go with Seastar
My opinion? I'd ditch the old Hynautic and go with Seastar. While Ryan is correct about parts still being available, they are dwindling.