When I raced Kinetic earlier this season (Olympic) the weather wasn’t so good, it rained a lot the day before and we were looking at the possibility of sustained rain at race time. We lucked out and it ended up being not too bad (a bit of drizzle while swimming, that’s all). I had to worry about the bike being in transition while it was raining. The first thing to say is that nothing is going to happen to your bike if it gets wet. For an overnight downpour, I would protect the saddle with a plastic bag, some saddles when they get a lot of water just soak up and stay humid for a while, and while you won’t notice at first coming from the swim, it’s a bit uncomfortable.
I would also put a large towel covering from your cockpit to the saddle, it will get very wet if it rains but it will minimize the amount of water that gets to your drivetrain.
While the bike will be fine, I would take extra precautions with the other stuff in transition on race day. It’s not fun to bike in wet shoes, same thing for running. So I would make sure that I at least start with dry shoes each leg. If it’s raining they will get wet eventually, but that’s ok. Plastic bags are your friend here. I come from a very dry climate and am not a big fan of rain, but when cycling or running wet I keep reminding myself that it’s just water. So I’d say just roll with it and don’t worry too much, you’ve got this.
I hope you have a good race!!.