Dear Sir,
Its nice to see the 'shraddha' (faith) in your father's Shraddham. To answer your question, it looks its not a straight forward answer, though the question is very straight. So, let me try to split your question and answer.
Is there any mantra difference in Iyengar shrardham?
As such, there will not be any changes to the 'rik's or mantras with respect to the rituals.
This is because, we follow certain sutra and this sutra is common whether it is for iyers or iyengars. Example, if you to a particular sutra, say, apstamba, then it doesn't matter if you are iyer or iyengar, but all will follow the same sutrakara's injunction or 'prayoga vidhi' for all their rituals. So, all it matters is, what vedam are you and especially what sutra you belong to, because same vedam has different sutrakaras like bodayana also belongs to the same krishna yajur veda as with apastambar.
(These rishis, apastambar and bodayana, are called sutrakaras as they deviced the rituals and procedures in a structured manner for us to follow. So, as a reverence and to mention the rishi parampara, we include their name in our daily abhivadaye namaskaram.)
Where could i find the mantras?
Honestly, this is the though question. Except for 'Apastamba' sutram, i have not found much resource for any other sutram. So, assuming that you also belong to the sutra, i will go on to answer.
1) Apastamba paarvana shraaddha prayogam ,
* Author - Srivatsa. ve. somadeva sharma.
* Mantras in Tamil and sanskrit (devanagari lipi)
* (unfortunately without intonation)
* Explanation in tamil.
Where to get - in giri traders (or) in sri surabhi publications, west manbalam.
Rs.100.
* Very informative and authoritative author.
2) 1) Apastamba shraaddha prayogam ,
* Author - kadalangudi natesa shastri
* Mantras in Tamil and sanskrit (devanagari lipi)
* (unfortunately without intonation)
* Explanation in tamil.
Where to get - in giri traders (or) in kadalangudi publications, T.nagar.
*Rs.110.
1) Apastamba shraaddha prayogam ,
* Mantras in grantha lipi
* (Fortunately with intonation)
Where to get - in giri traders (or) sri govinda dikshitar punya smarana samiti, kumbakonam.
Rs.150.
Hope this will help you to get an idea . May for other also who are looking for similar info.
Sarvam Krishnaarpanam.
Venkat.