I have a few files that have been executable before svn adding them. They have the svn:executable property set. Now, a few other files were checked in without the executable
You are right to use the svn property editing commands. The property is svn:executable.
To add the "executable bit" in svn
svn propset svn:executable on
To remove the "executable bit" in svn
svn propdel svn:executable
The SVN documentation for this is located here.
As far as not modifying the executables, you are not modifying the executable (a checksum will verify that), but you are modifying the SVN repository. Remember that SVN revisions file systems, not just files; so, a modification of the permission bits will increase the SVN revision number, even if it's just a modification of a file's properties (and not a modification of the file itself).