[enkeivette]

First, buy the tap for the stock size thread. See if you can clean it up and make use of it.

If not you can drill it for a helicoil if you don't need a lot of torque on it. And then lather the bolt up with silicone to prevent a vacuum leak. Let the silicone dry before you fire.

Otherwise drill and tap bigger, be careful if you're drilling into a head. Don't go too deep or too big or you'll make yourself another manifold vacuum port, or a sweet exhaust leak, or an even sweeter coolant jet.