Trump should issue a statement. “There’s only two things holding you back Timmie boy”. “Good judgment and fear”.
While I've said that Trump shoots too often from the lip, I think your above would make a good Tweet comeback. Of course, the MSM would have a field day calling it an implied threat, but the original threat of Kaine "punching" Trump would be seen and discussed around the entire World. It couldn't be buried.
I believe your suggested Trump reply would be seen as strength and power and resolve. Let's set up a "cage match" between Trump and Kaine. Ha!