I have a relative in the communications business who would tell you to throw it away.
The reason is - unless you are going to hook it up to a junk radio -there is no reason to use a junk antenna unless the radio is of no value to you.

Moores law tells me that it is probably past it's prime and it probably isn't going to last very long anyways.

A friend of mine has a antenna that sounds identical to the one you are trying to repair and the lengths on his antenna looks to be 12 inches.

I have not taken it down to physically measure it, but we are about to move it in a month or two and if you still need the exact measurement - I will get it for you.

I am surprised that the manufacturer didn't offer give you the dimensions.
Diamond X50 - radial / part number 07003
http://www.rfparts.com/diamond/x50a.html

73's
Jerry