I just downloaded IndustrialCraft 2 and was adventuring and couldn't seem to see any rubber trees.
Do I need to make a new world or can I somehow keep the world and make the new ores and trees spawn?
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You don't need to generate a new world, you just need to generate new chunks to be able to find the new ores and trees.
So grab your stuff and move to some new area where you've never been before. If you've been in this world for a long time the required distance can be quite large.
Keep in mind that rubber trees are far from the most common trees and might be quite uncommon to find even in newly generated areas. You'll be able to bring some rubber saplings back to your base to grow more, but the new ores won't appear around your base, so you'll need to make a new mine far away.