Sorry it is a "BO" not a "BU" What does the "BO" Desginate?
All the bodies were stamped with the last 2 letters reversed so they all got stamped BO or EO instead of OE in the serial # , the rad stamping will be there as well , often it is hidden under the Antifreeze decal on the top of the rad wall ,sometimes it is stamped into the rear face of the rad wall so you have to move the rad away from the firewall to see it . It will be stamper the same as the top of the cowl OBxxxxxx .
The dealer that originally sold the car may have paperwork on the warranty engine IF they are still in business , IF they kept the records & IF you can find out who sold the car originally , start the hunt !!