This IS native English, and it's perfectly acceptable. It's very precise, and there is no exact synonym. If you mean "do it now because you won't have another chance," then you already have your phrase. There are plenty of similar or related phrases, which can be used in slightly different situations. For example "do or die". But they mean different things. I can't think of a better way to say what this phrase says. So, let me give you some hard advice: stop looking for cool phrases and vocabulary, and pay more attention to the basics! I have spent all day working with Asian students who are memorising wordlists and poetry, but can't use the ten most common words in English correctly. And now I am looking at your question, getting very frustrated that you have the same problem. Why am I spending my free time helping you if you don't care enough to use the basic grammar you were taught in your first year of English? This may sound mean, but I am deadly serious. You will NEVER have a good command of this language until you start paying attention to sentence structure. If you want help from professional teachers, please take the time to proof-read your own work. You don't need me to tell you what's wrong with your question. If you think about it, you will see the mistakes. If you're serious about improving, you need to think about everything you do! I can't climb this mountain for you. At some point, you have to take responsibility for your own learning and USE WHAT YOU TEACHERS HAVE TAUGHT YOU. If you don't do it now, you will never do it. It's now or never! :)

I cannot think of an exact mimic of this expression. We do have (an extremely rude) expression which goes like this: "It's time to shit or get off the pot." The saying means that either you must achieve something now, or else you should give up and let somebody else have a go.

Act now or you will never make it.