Same as a dirbike or snowmobile crank, you need a big press. There are two ways to ensure the trueness of the crank. A jig is one, the other is to make marks and assemble as close to true as possible, and then use dial indicators and a set of v blocks, or on a lathe mounted between live centers. Then to correct it, you carefully tap it with a hammer. This isn't something you can do yourself very easily, unless you have the experience and the tools, but if you had both of those, you likely wouldn't be asking the qeustion in the first place, it would already be done.