Re: 1973 evinrude 18hp
No need to replace the head.
Take the head to an automotive machine shop and have them install a HeliCoil. It will be stronger than original. I have a 25HP that is running that way.
Make sure that you replace the head gasket with a new one, cleaning off the mounting surfaces and torque the head to the correct torque. You should be able to rent a torque wrench at an Aut Zone or buy an inexpensive one at Harbor Freight. Try to get a 3/8's drive that reads in in/lb, altho it is easy to convert in/lb to ft/lb.
By removing the head, you will make sure that no metal gets into the engine innards. And not to mention, it's a lot easier to handle.