Hi. So, looking at the OSM Wiki, Highway=Path is not for motorised vehicles. And Highway=Track has to be wide enough for a 2 track vehicle…
So what about ways that are OK for motorbikes to use but not wide enough for a 2 track vehicle?
Lots everywhere in the countryside of Thailand - with no restrictions as many locals are on motorbikes (scooters/mopeds). I would use a path to not mislead those wanting to transport by 2 track vehicle (e.g. 4WD pickup) and use footway, bridleway or cycleway if there are any restrictions or if the path is not suitable for motorbikes in some way.
If you look at the examples of http://wiki.openstreetmap.org/wiki/Tag:highway%3Dpath, you will notice that one of them has a mofa=yes. So feel free to use it, and add the necessary tags to allow access to moped/mofas and the like