Here you go!
Spray the area with a mixture of 1 part ammonia and 1 part water. They will vacate because of the smell. This also works on skunks, raccoons, and other critters. Spray regularly to keep them away. Spray your garbage cans too.
Best is to put a cover over the entries to your crawl spaces. Just make a frame and staple on hardware cloth. Possums love to go under. The frames can be rigged so that they can easily be removed just by unscrewing a screw or to lightly put in place to hold the screens in.
DON'T SCARE THEM, entice them. Place a treat where you want them outside & walk away & leave it there. Set a treat for them every day at the Exact Same Spot for a few days. They will get accustomed to it & even expect it after a few days. GET a live trap & place the same treat inside the trap. They won't think about the trap but WILL think about the treat. Once caught, transport them over 1 mile from your house. Cover the cage with a cloth so the animal can't see where it's going. Once you reach the drop-off spot, remove the cloth, open the trap & release them. Keep doing this until all animals are caught & released. Many remedies about deterring the animals usually don't work because a lot of people have tried them & the animals are desensitized to it. Opossums like leafy greens, veggies, cat food & tuna. Either of those would be a great selection for your treat. It'll work. We've done this for various critters.