I dont fast long since im already somewhat muscular and not overweight, but its a solid tool for me to lose some fat and stay leaner without losing muscle.
So from a guy who regularly does 48 hour fasts 2-4 times a month, i find i have way more energy about 15-30 hours into my fast for exercising. It seems i have less power but i can go for longer periods of time if that makes sense? I love working out during a fast.
I personally stick to 60-80% of my typical weight for lifting though just to avoid any issues.