Assalamualaikum,
Can a person who has missed lots of prayers just read Allahumagfirly instead of Dua-e-Qunut when he offers Qada of the Witr?

الجواب وباللہ التوفیق
Dua’a e Qunoot is not so long that someone would have difficulty reading it. Though it would be permissible in Qada Salat, however, one should not make it a habit, the reward would be reduced due to non-performance of the Sunnat.
واللہ اعلم بالصواب