Hi,
ja... 541 is a little bit old
I guess, this is not, what you wanted to hear, but I don't think, there is any possibility to upgrade to the last SVN. My advise would be: Look up your modifications, where they are, in which file and what they do. Write is down is something. Download a SVN and try to find the files and positions you made changes in your version and apply them as possible. That could be some work but in my eyes, the only way to do so.
OK, one more, but this will be much more work, because its the other way of modifying the files: You could go to
http://trac.roundcube.net/trac.cgi/browser and check every file, if it is newer than yours. If so, use the compare function to your SVN and apply the changes. But I don't think, that this will make you happier than the first idea...