Hi and welcome. That is an ancient piece of kit. I think you should look for collets first. I would be surprised if you found one. The one in the machine is so old I wouldn't trust it for a moment. Do that before you spend another dime on it. You may get it running only to find it is completely unusable. Old tools are not necessarily worth reviving, especially things like routers which spin at high speed. Don't mean to be a downer about this, but we constantly have people doing what you're doing who just cannot find a collet that fits. Porter Cable is not the same quality they once were, and old tools don't have soft start or speed control, other than an external power supply and rheostat to lower voltage. And that device is not usable on modern tools.
Give careful consideration to something like the Bosch 1617, which is a proven, reliable machine you can use for years, and which has a long list of useful accessories and after market items to enhance it.