Tums, regular strength, and the brand name. They work for everyone I've tried them on so far. Take at the first twinge. If that doesn't stop it, take another. Two have always done it for me. The effect is in seconds, not minutes. I have no idea why they work, but they do.

My calf cramping has always been a result of undertraining them. When I started doing sets of 30 one-legged calf raises a couple times a week, the problem went away and never returned. I focus on a full extension. But since you have a fused ankle, I don't get why that calf cramps. That foot would not seem to need the support of the gastrocenimius.

Google exercises for that muscle and you'll get a lot of hits.

Other than that, something to do with bike fit? Which again I don't get because of the fused ankle. Seems it would either work or not work. In these things, it's often the saddle being too high, so the calf doesn't get enough action, toes being always pointed. But probably not in your case, except of course that calf still not getting enough action.