If you must, cunnilingus and at the most a fingertip vibrator or gentle massage of her pussy. Some unspayed females, especially those of full/part Siamese and other Asian breeds, will direct you to the pussy when in heat and petting/brushing them or will hump/grind on you.
Penetrative sex or using anything larger than a fingertip vibrator or a thinner finger by itself, is a disaster waiting to happen. And not all lady cats will tolerate being handled down there.
I have a decent amount of experience with female house cats