You should be OK
. According to ANA's webpage, you need a minimum of 60 minutes for an international -> international transfer at Tokyo/Narita. This assumes that:
1. Both your inbound and outbound international flights are in the same terminal (ANA uses T2
until June 2006, T1 thereafter, code-share flights may vary between T1 and T2
2. Both flights are on the same ticket/reservation.
3. You have through-checked your baggage to SIN
4. You do not otherwise have to go through Japanese immigration/customs procedures at NRT
(if you were traveling on separate tickets, you might have to do this).