First you need to measure the fitted height of the springs. That's the length, top to bottom with the valve closed.
If you're still in ChCh take your springs - with retainers & spring seats - to Kelfords.
If they are feeling kind, they can measure the pressure at fitted height and possibly find you an off the shelf replacement.
I'm guessing but I'd be looking for something around 70-75 pounds pressure at fitted height. More than that and you risk wearing out the cam and followers
quite quickly.