Literally just went at it with a flat file and a rounded file to do the round at the end.
Best bet is to mark 2mm up from the top of the dropout at the opening of the dropout and at the end and score across so you've got that flat line to stick to. Same for the bottom to get it to 14mm.
Bmx hubs have 110mm spacing (How long it is) and MTB hubs (What your frame is made for) have 135mm spacing. Meaning that by putting a bmx hub on it and tightening it up propperly bends your frame 2.5cm so it's highly likely it will snap