I think the decisions you make should be based on the severity of your disease. For instance, I have chiari with syringomyelia - it is because of my syrinx that I choose not to do things that will "jar" my neck like roller coasters and horse back riding. But - even though it is probably more dangerous - I still ski. I just can't give up EVERYTHING I love. I figure, if I want to be careful, I'll stay in bed all day!! But I'd rather keep living. If you love to ride motorcycles, then I say ride, but be smart, where a helmet (something lite that won't sit too heavy on your neck)
Also pay close attention to your symptoms. For instance, I have balance issues and you couldn't pay me to ride a motorcycle right now. Or ski for that matter - not that I could ever ski anyway :).
I don't think by itself it causes weight gain, but it can limit your ability to exersize - so by default you can gain weight if you don't cut back on your food intake. I've gained about 10 lbs since my surgery in April.