What sentence parts needs to be repeated here?

Solution 1:

If you want to be clear, only the first one will do. The other ones could be misinterpreted as We need to find out how to lower the costs or need to produce more.

Solution 2:

I agree with Guffa. You want to have a parallel construction, but unless you repeat the "how to" part, it will be unclear which parts are parallel.

This sentence makes it clear that the "how to" parts are parallel:

We need to find out how to lower costs or how to produce more.

The parallel in these sentences could be "to find" and "to produce":

We need to find out how to lower costs or to produce more.

We need to find out how to lower costs or produce more.

Alternatively, you could clarify the parallelism in the last two constructions by using "either":

We need to find out either how to lower costs or to produce more.

We need to find out either how to lower costs or produce more.

Solution 3:

Disregarding grammaticality, I don't like any of OP's possibilities. Better would be...

...how to lower costs or increase production.

...which juxtaposes the two alternatives more clearly.