IIRC, the CX500 models that came with non-vacuum petcocks have 16mm threads but the GL500, CX/GL650 and later CX500 models that came with vacuum petcocks have 18mm threads. Hopefully someone who knows for sure will either confirm or correct that.
If you convert to a non-vacuum petcock you will also need to cap the barb on the left side of the right carb where the vacuum hose connects.
FWIW, I have a bunch of old petcock parts and could send you what you need but with postage rates being what they are a new one from China would probably cost you less than postage from Canada, not to mention that there is no guarantee that the one you have will work - there is a rubber "donut (actually a disc with 4 holes) in the main valve of the petcock that can deteriorate with age & use plus the vacuum part would probably need to be rebuilt (you can get a kit for that but the donut is not available.
Welcome to the forum. Please add your location to your profile and your bike's model and model year to your signature so that you don't have to remember to tell us every time and we don't have to keep asking when you forget (see Forum Settings link in my signature).
And welcome to the world of antique vehicle ownership (they own us, not the other way around). Your bike is about 4 decades old and the Previous Owners may or may not have done the maintenance necessary to keep it safe & reliable so it is highly recommended to download the Factory Shop Manual for your model (available through the CX Wiki - link in my signature) and go through all of the service procedures, regardless of whether your bike has reached the specified mileage.
I also recommend looking on all rubber parts with suspicion because rubber does not age gracefully. Check the date codes on your tires and replace them if they are over 5 years old no matter how good they look & feel because old rubber simply cannot flow around the irregularities in the asphalt well enough to grip, especially if it is cool or wet. If your bike still has the original rubber brake line(s) (should be replaced every 2 or 3 fluid changes = 5 or 6 years) I recommend shopping for modern stainless braided ones (they last practically forever and double the life of the fluid).