I would try to hold the chick's bottom gently under warm running water for several minutes to begin to soften all that dried on poop.  That way you can very gently try to remove it once it's softened, but you don't want to pull on anything hard and crusty and risk further injury.
A chick that age may also have a slight scab on their navel that looks very similar to what you are dealing with, and that would be quite fragile and delicate, so you need to use care not to pull on that and kill the chick.  Once you feel you've made some progress, if you need to give the chick a break and let him / her dry off and warm up again under your heat source, you could do that as well, so as not to overtax the baby chick.  But you have to get all that dried crusted poop OFF THEIR VENT and make sure the chick is eating and drinking.
You can also try offering some plain yogurt as well, as a treat / supplement to provide probiotics, or a supplement in their water if you have that available.
Please let us know how it goes!