Why are you so stuck on just changing the springs?
That's not a linear first step in engine performance building.
Stiffer springs on an otherwise stock engine are only going to add resistance to the valvetrain, inducing drag and lowering performance. The reason for stiff springs is a side effect of much faster RPM and greater cam profiles to follow.
I've seen and read about WAY MORE rod failures than I have flywheels that grenade.
These are industrial engines made to run at a constant RPM at like 3500 all day. They have low compression, barely adequate oiling systems, and poor airflow.
From what I've seen, a billet rod is a must. They have better oiling and a real bearing surface.
The best HP gain per dollar is going to be airflow. An engine is just an air pump. Make it more efficient. A slide carb, open exhaust, and some easy port matching will bring free ponies.
Head work like port cleaning and deck milling will bring a few more.
This is just labor-intensive but costs me nothing as I have the ability to do it myself.
A stock cam in my opinion is the best for low-end performance. Its lobe separation angle provides the smoothest idle.
Bigger cams work better at the higher RPMs, but the trade-off is from the low end.
Some ratio rockers would be a small upgrade, making the valves think the cam is bigger. They would provide a little more duration and lift.
Even then, without a large cam, I think the stock springs will suffice.
You came here asking for advice from humans and not AI. We have all basically said the same thing, yet you're still determined to change springs. In the end, it's your engine, and do what you will.
As I've asked before, what's the end goal? Is this a drag motor? Race kart? Yard kart for the kids to bomb around in?
I run stock cam and flywheels in my engines that get held at WOT for hours. Billet rod and head work is all I do because I'm a cheap 🫏. I have yet to blow one up.
