Stiffer springs than stock = better???

Apparently in Japan, you can get a 3rd party arb

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Just a front ARB on a JImny. Be aware that fitting a stiffer front ARB will increase the cars tendancy to understeer.

As others have said, valve springs alone won't help with anything. To get more power at higher rpm you will need more aggressive cams. There's no point changing the valve springs until you have some cams to fit and a programmable ECU to tune the engine accordingly. But you have previously mentioned you are going to be fitting a bigger engine anyway, so put the parts into that rather than the 1.3.