US export certification controls is very tight. US blocked the
Phalcon radar to China. US blocked the AESA variant of the Israeli
EL/2052 radar for the Indian LCA.
What US tech, Thailand put onto Chinese frigate would have
cleared thru US export licenses. Same as the gulf
states+SA. What Pakistan put onto their F-7s would have been
European, rather than US avionics.
Some are able to utilise US tech despite controls eg Iran after
the fall of shah but this has not been particularly successful or
optimised. Many countries with dual Russki + Western tech
all face integration problems.
Western a/c such as Typhoon or Rafale would also have required
US clearance. No issue for Singapore, that's why those were
considered. That's why its major that US pulled some export
clearance for the gripen for the Indian MMRCA contest. That's also
why Su-35 never made it into the top 3 for the SG comp (they won't
give detailed access to info).
Miltech controls are very strict. If SG violates US export
controls, one can be sure no further access (at least to the
top-end of the tech range) will be granted. Won't happen. J-20 or
T-50 won't happen either. Its either F-35 or F-15SE.