Don't confuse the size of the bolt head, with the size of the threaded shank of the bolt. A set of cheap metric thread gauges will save you lots of heartache. You can find the thread gauges on e bay. You use them by selecting one leaf of the thread gauge and laying it's serrated edge along the thread of the bolt shank. If the serrations and the bolt thread mesh completely when you hold the gauge up to the light, you can then read the thread size which will be marked on that particular leaf.
Once you have determined the thread size, you'll need to assess the state of the stripped thread in the crankcases. If the thread isn't too badly damaged you may be able to use the original thread size tap to clean up the threads and make good. If you can do this coat the tap with grease so any swarf formed by the tapping will stick to the tap and not find its way into the engine. Wrapping the drain plug threads with a couple of turns of PTFE tape can also help with tightening up slightly loose meshing threads
If the thread is too badly stripped you might wish to replace it with a Helicoil insert, but this will be a complete pain in the ar*e, trying to drill out the stripped thread and insert the helicoil while laying on the floor working underneath the bike. You would have to ensure the drill bit was perpendicular to the hole at the same time ensuring no metal enters the crankcases and you don't drill too far. Not for the fainthearted by any means, I certainly wouldn't fancy it myself. To be completely honest I can't see any machine shop being willing to attempt it with the engine in situ, but you never know.
Oh the usual pitch of metric fasteners used in the automotive world is 1.25 and stick with the OEM'O' ring . Good luck.