Cool project. Tip: acetone will readily dissolve the plastic parts of the card leaving the antenna(s) and chip intact. I believe a few other people have made drop-in PCBs with NFC antennas. Here’s one: https://n-o-d-e.net/datarunner.html

Also, no F-91W thread would be complete without a mention of the sensorwatch project: https://www.sensorwatch.net/