Tankless water heater - putty/ caulk

[ShadyLedge]

Needing to pull out the Girard Tankless Heater on my SS Agile. Need to swap about a leaking heat exchanger. Trying to figure out what caulking I need to use when I reinstall the unit? From what I can tell need to cut the old out. Remove the unit and recailk or putty. Any one have experience with this? Thanks