In the first Wikipedia example of the Old Novgorod dialect https://en.wikipedia.org/wiki/Old_Novgorod_dialect both little yus ѧ and the new я is used. Is it just a random spelling difference? Do I read them in the same way? And if yes, is it "ja" or "ę"?
As the illustration shows (as well as the presentation at http://gramoty.ru/?act=full&no=109), the symbol rendered as "я" is in this case actually an ordinary "а". Per my knowledge, the loss of nasals was a general Eastern Slavic phenomenon of the first half of Xth century ([ę] > [ä]), so any little yus shouldn't be treated as anything but the spelling of ä. The author's spelling choice of възѧлъ as възалъ I cannot shed light on; per http://gramoty.ru/dnd/a14.pdf p. 258, that is likely the author's misspelling.
[Note also the conclusion ibid. (p. 259) that the text 'shows no univocal phenomena of O. N. dialect proper; furthermore, several details (though not conclusive) might set it out of the mass of XI-XII cent. gramotas. Possibly, the author, Zhiznomir, was a native of Southwestern Rus'. Perhaps the example in Wikipedia should be replaced by a clearer one?]