この文は自然ですか？

不自然です。if が抜け落ちているのではないでしょうか。
You can just look up the words if you want to.

すみません。自然ではないと言えるでしょう。問題点は"[...] you want to". ですがその以外は自然でしょう。
Sorry, it may not be very natural. The problem is in "[...] you want to". Aside from that, it is natural.

話す時に、次の例の方が自然的かもしれません。
When speaking, the following would be more natural:
- You can just look up the words you want.
- You can just look up the words that you want.

When writing in a formal context, you would not want to use either of those. If you are interested in that case I can provide more information.