I did some checking on eBay prices for Jap makes a while back and even in good condition I didn't see a Thunder 1A get more than 200 Euro. If you look at what's gone recently, it's the same. A nice black 1A went for 156 GBP. So, to me, it wouldn't be worth the effort of the project, unless you can put something special into it.
Might be worth asking on basschat.co.uk if anyone has one they can trace for you, or even a dead body.