Building a 12 row from parts is not that difficult mathematically, but really depends on your level of expertise and shop equipment available. I have built a couple for my own use. The single most important factor is to have the toolbar absolutely straight. The hex drive shaft must slide in relatively freely. It can not bind during operation. Also that hex shaft should come from a head of like row spacing. The gear box engagement areas of hex are heat treated. Plain cold drawn 1-1_8 hex will work for a while ......... Your best bet is to order new drive shaft from parts. One other area of importance is the frame braces added to larger frames. The exact location is not that critical [must not interfere with drives or shields], but to have them fully welded to frame as the braces absorb the vertical and horizontal forces as head goes thru the field. If need hints on auger splice ask question on forum. This cornhead project is not that difficult, and you can save $$$. Good luck!