I just finished restoring another Tanegashima or Teppo, and I can tell you that the action of this kind of snapping matchlock doesn´t require a tough spring at all. In fact it should almost be as soft as possible.
The falling of the match into the pan should be very, very soft and weak, although not slow, so as not to just crush the glowing matchcord and make for a missfire.
In all the japs Tanegashima I´ve handled, I found no one that had a strong brass spring.
I´d say you should hotbend it. Gives you a more controlled bend and a nicer looking result.
And you need not hammer that brass spring after forming it. The movement of the match arm on a Tanegashima is very short, so there will be not much strain on that flat spring, and the tension will be more than enough, without hardening it.
If you´d like I send you some pictures on the gun I just restored.Had to build the complete lock mecanism and the pan and cover, since this gun was turned into a percussion arm, with a crude, old Enfield lock.
But since I dont know how to add a picture here, you have to send me your ordinary E-mail adress.
Best from Sweden
Magnus Wiberg