There are already plenty of F16s on the way to Ukraine committed from ex-Euro (Netherlands, Norway, Denmark) stocks, 50-60 of them AFAIK. The limiting factor is Ukraines ability to operate them.
I guess the F16 C/Ds would be a good addition, if US politics permits.
The US retires advanced aircraft every year, back when this war started or before, we could have sent retiring aircraft destined for the Boneyard to Ukraine, here we are over 2 years into this war and the F-16s might get used this year.