Is there a complete English translation of the Narasimha Tapaniya Upanishad available online?
You can find English translation of all the 108 Upanishads at Vedanta Spiritual Library.
Nrisimha Tapini Upanishad belongs to Atharva Veda, here are links to the related Upanishads you might be looking for:
You can get/download all the Upanishads related to AtharvaVeda in a portable document format.
You may also find it on vedarahasya.net.
Translation of Nrisimha Tapaniya Upanishat Uttara and Purva both on this website- Nrisimha Uttara and Purva Tapaneeya Upanishat