Rebuildable cores of the 235-sized YFs are fairly expensive. YF Cores for the 216 are more common on eBay and seem to have much more reasonable asking prices. You could also look for one in the swap meet section here.
Now that we know you have a hybrid of some sort…might as well just get a 964s, rebuild it, and know that you have the right thing. Sounds like your engine ran well before all this.
