I read recently on the FE-FC Enthusiast Facebook page that a HQ window regulator spring is a suitable replacement, if you can't find an original one. Haven't tried it, but I daresay they're pretty easy to find. An image search shows a spring that looks similar (although the outer hook looks quite long - perhaps that doesn't matter once it's tensioned?)
Anyway, that's free advice, worth at least what you paid for it
If you're after an adjustable boot pin kit, PM me, I have some at the moment.
cheers
RET