How does one alter a recipe that calls for one fruit with another fruit of differing water content?
There's no trick, there's math. 100g of banana is about 75g of water (1g of water is 1ml, so easy to measure), 12g of sugar, and 13g of fiber and other stuff. A pumpkin is about 92g of water and 3 grams of sugar, leaving 5g of other stuff. 

My banana bread recipe calls for 2 medium bananas, that's about 250g of banana. That's 188g of water and about 30g sugar. If you add 250g of pumpkin instead that's 230g of water and about 8g of sugar, so pumpkin would add 42g of water (42ml, a bit less than 1/4 cup), and take 22g of sugar. That's enough of a difference to really throw your recipe out, so you'd want to remove 42ml of water, milk or other liquid and add 22g of sugar (4g of sugar is about 1 tsp, so 22g is about 5.5 tsp, or just under 2 tbsp).