You don’t tell us where you’re located or if proximity is an issue…

A quick search on the Barnstormers website turned up:

“CURTISS JENNY PARTS/AIRFRAMES • AVAILABLE FOR SALE • Museum quality Curtiss Jenny parts made to order. Wood work in spruce, ash, ply, all sheet metal work in MS and Aluminum, custom investment castings and fittings produced for your project. Any aircraft parts available by request. Hard to find materials and castings made to order. SolidWorks drawings made to order. 3D prints for patterns available from us. Over 30 years experience. OX-5 and Hisso Engine overhauls available, Dixie, Berling Mags over hauled to new. Irish linen and synthetic covering services offered. Sheet metal cowlings made to order. • Contact John Gaertner – BLUE SWALLOW AIRCRAFT, LLC, Owner – located Earlysville, VA USA • Telephone: 434-295-7177 • Posted June 13, 2015

Likewise in Trade-A-Plane:

JN4-D Open shop time for antique restoration.
A number of basket cases JN4-D (OX-5) Jenny
Contact Information:
Virginia Avaition Co/Wright Experience Inc
(540) 347-1909
Fax: (540) 349-1764

Contact: Ken Hyde
Address: 7099 Glenn Curtiss Lane, Warrenton, VA USA 20187

I don’t know if they have complete engines, but anyone who rebuilds Jennys must know something.

You don’t tell us if it MUST be an OX5 or Peugeot.
When you need something that is rare/expensive or unobtainable, think about alternatives.
Jennys used other engines than the OX-5, so would an OXX-6 or Hisso be acceptable?
If they were close/good enough for the aircraft in period, would they be good enough for your purpose?