I look at headphones like my wife looks at shoes.
The more the merrier and a changeup is good for the soul.
First off, if you can get replacement parts for your Fostex for a reasonable price, go for it.
Second, I'd still go for a new pair just because I can. See line 1.
I don't know about the cord lengths, but I think you might appreciate a pair of Grado's, perhaps the SR-60's which, coincidentally, seems to be their going price. Sennheiser HD-280's for about a C-note are also a good value for enclosed phones.