Fun is in the OED and so we will continue to use and accept. In the West coast of Scotland at least it is perfectly normal. There are many examples of English in the course that could be regarded as non standard as we are trying to mirror the Gaelic to avoid confusion. I am not sure these alternatives really convey the meaning of the Gaelic.
Fun is a noun so it needs to be 'such fun' or as Jean says, 'so much fun'. You cannot be so fun. Not even for the American dictionaries. https://www.grammarphobia.com/blog/2014/07/so-fun.html
As Ciaran said above, 'fun' is listed as an adjective in the Oxford English Dictionary, with its earliest listed usage in 1827:
1827 C. Lamb Lett. (1935) III. 89 It would be rather fun to see his odd ways.
Merriam-Webster also lists 'fun' as an adjective, and given that these two dictionaries set the standard for British and American English respectively, I think we can agree that 'so fun' is a perfectly acceptable phrase.