This is quite interesting.
In the a Q&A discussion on MSN (Japanese) about this topic ( http://questionbox.jp.msn.com/qa3556938.html ) three out of seven people responded with brief answers, claiming that 生きんが is contracted from いきぬが.
This is interesting because ぬ is a negative conjugation; but of course grandfather didn't live well so that he would not reach 100.
Answer no. 2 claims that 「生きぬ。意志を表す言葉では。」("It is a word expressing intent.")
According to the Wictionary, ぬ is not only a negative marker, but also an archaic indicator of completion:
Thus the hypothesis is: いきるが → いきぬが → いきんが.
If we search for いきぬがために, the only hits are multiple choice exam questions where this is given as a distractor against the いきんがために answer, e.g.:
Q: 人間は（ ）、いやな仕事をしなければならないこともある。
Presumably, the correct answer is B. Even if B is derived from A, that form isn't used. (Possibly because it invokes the archaic sense of ぬ noted in the Wictionary? This could be a case of the vestiges of something archaic which survives in an altered form only.)