FlyinHawaiian

Delta operates from two terminals at JFK; Terminals 2 and 4 and the JFK flights are operated by smaller regional jets. The flights will either arrive at a regional jet gate at the end of Terminal 4 (likely) or Terminal 2 (less likely). Regional flights tend to be among the first to take delays if the crowded airspace in the Northeast Corridor gets too congested. Also, DL does not normally interline baggage to non-partner airlines on separate tickets; this means if you ask DL at DCA to check your bags through to SIN, they will likely say no.

If Terminal 4, the walk from exiting the plane to baggage claim is about 15 minutes. There is a shuttle bus (JFK Jitney) operated by DL that one can take from the far end of T4 to a gate closer to baggage claim, but it will not save much, if any, time. You'll then wait for your bags to arrive at the carousel and then need to go upstairs, check your bags (before the cutoff) and then re-clear security (without PreCheck/Clear or a priority boarding lane, this can take 30+ min).

If Terminal 2, it's a very small and compact terminal so the walk to baggage claim is no more than five minutes. But, you will still need to wait for your baggage, then walk out of the terminal to the AirTrain (about five minutes) and then get to T4.

Bottom line - I would give myself ample time to do such a transfer and I don't think two hours is enough.