Would it be correct to say,
"Well then, I shall be on with it, now shalt I".
Would it be correct to say,
"Well then, I shall be on with it, now shalt I".
It's pretty old-fashioned, but sure. More modernly, you could try "Well, then, I'll get on with it."
Don't use "shalt." I don't know Early Modern English well enough to be sure if that is proper usage, but I bet it isn't, because it sounds wrong. Ending with "I shall be on with it" is perfectly fine.