My plastic header tank on my 94 classic wrx is leaking will a 2nd hand metal one of later classic be ok and fit?

 

You should be ok, as long as you wash it out first, and put fingers over holes to test for leaking.
If I rember a conversation I had at ace cafe a few weeks back, plastic header tanks did have a problem, the classic and newage ones (i had a plastic newage one, but now replaced with a Forge alloy tank) that's I think why even the new ones have a metal header tank.