You're going to have to change either the spur or CB so you have more room. I think I'd go for a small spur, since that will lower your gear ratio further. With the .18 I doubt it's going to get low enough to really hurt your acceleration. I'm not sure how many teeth you'd need to drop.. easiest thing would be to go to your lhs and compare the sizes with your current one. I'd say you'll need to go at least down into the fifties.