Here are some versions I tried for the ending, written mostly on New Year’s Eve.  Actually, these are only a tiny portion of the different versions I tried, because I tinkered with each version for a long time.

Possibility #1:

So if you ever see a jolly white-whiskered gent ringing a bell by a kettle, or if you see a merry gnome in red at the mall, remember:  Mr. Claus has many helpers, but only one man knows every child’s wishes on Christmas Eve.

He’s Santa.  He’s the one with the magic.

Possibility #2:            

So if you ever see a jolly white-whiskered gent ringing a bell by a kettle, or if you see a merry gnome in red at the mall, remember:  Santa has many helpers, but there’s only one man who knows the heart of every believing child on Christmas Eve.

He’s Santa.  He’s the one with the magic.

Versions developed in discussion with my wife Joyce the next day:

So if you ever see a jolly gent ringing a bell by a kettle, or if you see a merry gnome sweating in a red wool coat at the mall, remember that Santa has many helpers,

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but only one man has the magic.

The next version has a faulty sentence structure of “you’ll see that…but.”  You can’t see a ‘but.’:  So if you ever see a jolly gent ringing a bell by a kettle, or if you see a merry gnome sweating in red wool at the mall, you’ll see that Santa has many helpers,

but only one man has the magic.

This is the one Joyce liked best (it's almost the final version, but not quite):

So if you ever see a jolly gent ringing a bell by a kettle, or if you see a merry gnome sweating in red wool at the mall, remember—

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only one man has the magic.  He’s Santa.

Other possibilities:

Mr. Claus has many helpers, but there’s only one man who rides the skies with a gift for every child on Christmas Eve.

who rides the skies with a gift for everyone.

man who knows the way to every house on Christmas Eve.

Possibility #3:

So if you ever see a jolly white-bearded gent ringing a bell by a kettle, or if you see a merry gnome in red at the mall, he might be the real Mr. Claus.  And he might not.  But whoever he is, there’s only one man who rides the skies with gifts and reindeer each Christmas Eve.

He’s Santa.  He’s the one with the magic.

Possible version #4:

So if you ever see a jolly white-bearded gent ringing a bell by a kettle, or if you see a merry gnome in a red coat at the mall, he might be Mr. Claus.  And he might not.  But somewhere in the world, in a place where the children truly believe in him, there is a real Santa. 

He’s the one with the magic.

So if you ever see a jolly white-bearded gent ringing a bell by a kettle, or if you see a merry gnome in a red wool coat at the mall, he might not be Mr. Claus.  But somewhere in the world, in a place where the snow is bright and the carolers sing, there is a real Christmas hero.

He’s Santa.  He’s the one with the magic.

So if you ever see a jolly white-bearded gent ringing a bell by a kettle, or if you see a merry gnome in a red wool coat at the mall, he might not be Mr. Claus.  But whether he is or not, you must always believe that somewhere in the world the real Santa is ready.

He’s the one with the magic.

So if you ever see a jolly white-bearded gent ringing a bell by a kettle, or if you see a merry gnome sweating gallons in a red wool coat at the mall, don’t forget:  there’s only one real Santa. 

He’s the one with the magic.

Possible version #5:  Joyce liked this one:

So if you ever see a jolly white-bearded gent ringing a bell by a kettle, or if you see a merry gnome sweating gallons in a red wool coat at the mall, don’t forget:  Mr. Claus has many helpers, but there’s only one real Santa.

He’s the one with the magic.

Possible version #6:

So if you ever see a jolly white-bearded gent ringing a bell by a kettle, or if you see a merry gnome sweating gallons in a red wool coat at the mall, don’t forget:  Mr. Claus has many helpers, but there’s only one flying gift giver who circles the world.

He’s Santa.  He’s the one with the magic.

So if you ever see a jolly white-bearded gent ringing a bell by a kettle, or if you see a merry gnome in a red wool coat at the mall, don’t forget:  Mr. Claus has many helpers, but there’s only one real Santa.

He’s the one with the magic.