A number of the 18th century New England tunes have been kept alive by the Sacred Harp singers. The Resource list has details on 2 sources. An American Christmas Harp, compiled by Karen Willard has a mixture of 18th and 19th century music. I'm pretty sure the notes in the book give the date of composition. The web page says it's out of print, but it is available for $16 + shipping. You can link to e-mail to the compiler & ask her for details about specific tunes.

I don't own Awake To Joy! Christmas Carols for Part-Singing, compiled by Nym Cooke. Karen might be able to tell you how much overlap there is between her's & Nym's book.

A couple from the tradition that come immediately to mind are: Joseph Stephenson (English I think) Milford (If angels sung a Savior's birth)

Daniel Read: Sherburne (While Shepherd's Watched their flocks by night)

I think Billings set the same text, found in Christmas Harp.

Isaac Watts wrote some great words: Shepherd's Rejoice lift up your eyes. Both tunes in the Sacred Harp are late for your performance, but I think Oxford is in the 18th century style. It's Common Meter, so a Billings or Read CM tune could also be used.