Mein nichtmuttersprachliches Empfinden sagt mir: Keep on - aber vielleicht spukt mir da die Formel 'keep up the good work' im Hirn herum?

It's hard to say, because I don't think I would use any of them - I think I would say something like:

All start? Seriously? No, we shouldn't ever stop...

Edit: I've changed my mind.

I think I could say: Start? No, keep on.

Ich würde sagen "keep going". Bei den anderen Varianten fehlt mir etwas die Erweiterung.

Or, for instance

"Start? Are you serious / Are you joking? You mean keep going."

Edit: actually, I think I prefer "carry on".

For a single verb answer, I would say “Start? No way; — continue!”