sorry about rambling on. For reboring the long hole, your best bet is a tapered reamer to get the cutting action going without totally beating the hell out of the machined edge, but a modified drill bit will work, but by the time you go through all of that I think you can buy one of those intakes. I don't recall mine as being too expensive. A heck of a lot less work. The angle of attack is a bit hairy, especially if you use a handdrill.
When you measured the bore you said you got 14mm. BTW, did you use set of calipers? Chances are, the hole is tapered because the rod that made the hole during the casting process was tapered, so it could be pulled from the casting once it cooled. The bore at the elbow might be 12mm.
The ideal intake would be slightly "s" shaped, but that would be cost prohibitive due to the unusual shape and drawing any cores.