information about tubes?
Unless you are particularly heavy, I would not worry about size - not that much difference in load carrying between a 2.1 and 2.4 that it will cause a blowout. Blowouts are caused by a tire failure of some description (bead failure, not seated on rim properly or cut to tire are the most common causes). You say "old tire and tube" - how old and what condition?   

If your choice is ride on the 2.1 or not ride - I know what I would do.