All three.

Start looking for a new position. Things could go well, but if they start going badly, you want to have options outside the company.

Start fishing for new opportunities within the company. If you're not also leaving, there must be something there worth keeping. So, if it looks like you're not on a project that is good for you, another project could be a better hole for the you-shaped peg.

Start talking to your manager. If they haven't cancelled the project yet, maybe there's something going on behind the scenes. If you're going to be the senior employee on the project, you should have the pay, title, and fancy business cards to match.

But do all three concurrently. There's no reason to let any one of those interfere with the others.