I have just started working on my 1973 CT3 and I just noticed that my front wheel is 21 inches. The bike has a high mount front fender on it.  Will a standard front fender fit with a 21 inch front wheel? I don't like the look of the high mount fender and want to return it to a stock look. I see that a 21 inch tire is about 3 inches bigger than an 18. Thanks!

Yes, a standard front fender will still fit BUT the space between the wheel and fender will be a lot less. If you are just riding on the street there should be no problem. However if you plan to ride a lot in the dirt there could be some problems ESPECIALLY if you ride in some mud.

Maybe I should look for a 18 inch wheel. I appreciate the response !

One thing I did not mention, but should have, is that a 21 inch front wheel will definitely ride better on dirt trails. A 21 inch front will go over obstacles on the trail much better than an 18 inch front wheel. I put a 21 inch front wheel on my CT3 for trail riding and it was a noticeable improvement.