Want to replace front tire with a21 inch tire, but noticed that the 21 has a 3/4 inch Bering and my original has a 1inch shaft ,can I put a 3/4 inch on the bike and will it fit ok/

If you are speaking of a 21" tire then you need to get a Wheel that is for a 21" tire. If you have the clearance and the wheel has the same shaft size should work okay.