Bruce is using what I would use. It will fill out the mudguard and in my opinion, improve the handling. I had the same set up on a Black Shadow. The previous owner put the 21" on the front in 1960 because he liked the feel of the larger front wheel.
There are lots of modern 21" front tires available as they are used on many modern bikes, from cruisers to dual sport bikes. I would use an Avon on the rear also.
David