Since SIN is now considered "clean", when SQ26 arrives into FRA the continuing pax on FRA-JFK are directed into the second floor straight into the departures area (B non-Schengen, where AC also flies out of) with no security check required, while terminating or connecting pax are sent up to the third floor to immigration or transit security. Transit security at that hour should be pretty quick and it deposits you right by the AC MLL.

However every time I've flown SIN-FRA-JFK with SQ upon arrival in FRA I've never had my BP checked to make sure I'm actually continuing on to JFK. Hence it's conceivable you could just follow the signs for JFK connecting pax, skip the security check entirely and just go straight to your AC departure gate. Really there shouldn't be any issue since SIN is a clean arrival, but of course YMMV.