If you don't get one from Tamiya, let me know exactly which part you mean, and the part number as shown on the instruction sheet. If it's the windscreen, then the Mk1 used the screen with external armour, and the MkV used both this, or the internal armour on later versions. I might have an external armour from a different kit, but not sure without checking.
However, Tamiya should send a replacement without problems.