Roberton was a Glasgow man born and bred, but it isn't compulsory for a Scot to use "standard" Scottish spellings (probably only a minority do that); and if he or she doesn't, it doesn't follow that they are writing "in English". Scotland, like any other country, has a wide range of accents and dialects, and many pronounce the word closer to "own" than "ain" in any case. What spelling system is used in Scottish schools nowadays? Are children taught to write "ain" or "own"?

The verb form is spelled "own" in Standard Scots (as defined in Scottish dictionaries) anyway, so it's not an important distinction. I do agree, though, that it is important to distinguish between "ain" and "ane", as they have completely different meanings; though Scots "ain" and English "own" are just the same words spelled differently in order to reflect differing pronounciations, as are "ane" and "an". Scots and English both consist of a series of overlapping dialects (which do not respect national boundaries), and the apparent gulf between them is largely the result of the artificial way in which standard forms have been defined over the years.