I had the same issue about four years ago and successfully helicoiled using a 22.5mm drill to clear the hole, then a M22x1.5 tap. I can't see what size the actual helicoil is as the label on my packet has deteriorated but there are tables online that will give you that information. But these sizes enable the original filler plug to be used. I remember that I had to do a lot of research online to find the correct elements.
If you were closer I'd lend you my drill and tap as they're expensive for what is likely a one time use, but maybe you've already got that level of equipment. What I can do, if you have any problems working out the helicoil, is send you one of mine. I had to buy a packet of five, so I have four spare. Best of luck.