Hi Neil,
If you are going to use a commerical water jacket, it is always best to machine the cylinder after you get the jacket. this way you can cut the cylinder to fit it. Here is a jacket at auction on Ebay. It will work if you didn't cut to much. It is held by two screws and has gaskets at the top and bottom. At least it will give you an idea of how to make one.
http://cgi.ebay.com/Zenoah-260-Water...QQcmdZViewItem

Dave