Dave, you've got an early 'pre fastback' Sprite/American Eagle frame with a DT1 Yamaha motor. I hate to spoil your excitement but the forks aren't Ceriani's, they're either REH or more likely Sprite forks which were a cheap knock off of REH. The best way of knowing is by pulling the triple clamps off the bike and seeing if they have a proper head stem (REH) or a bit of threaded rod (Sprite).The front hub is REH or possibly some Italian thing or other.
The Yamaha motor is a vast improvement on the Sprite engine that probably lived in there and probably better than the Villiers Starmaker too
. The problem with this engine swap is that the Sprite frame is a real heavyweight, much heavier than the DT1 frame, much like when I fitted an XL Honda motor into a Montesa King Scorpion lardmaster, way heavier than the original XL unit.
I hope you didn'y pay too much mate