>+              (uri 
"mirror://sourceforge/weka/weka-3-9/3.9.5/weka-3-9-5.zip")

These isn't source code, these are binaries (*).  With the exception of 
the bootstrap seeds, things in Guix must be built from source (and even 
then, we still can build them from source, with "guix build 
bootstrap-tarballs").

Looking at the git repository, it appears to use Ant, and we have an 
ant-build-system, so unless there are complicated dependencies, it 
should be feasible.

 >+    (native-inputs `(("unzip" ,unzip)))
 >+    (inputs `(("jre" ,icedtea)))

Nowadays you can avoid input labels with (native-inputs (list unzip))
and (inputs (list icedtea)).

 >+                           (display
 >+                             (string-append
 >+                               "#!/bin/sh\n\n"
 >+                               (assoc-ref inputs "jre") "/bin/java -jar "
 >+                               out "/share/weka/weka.jar \"$@\"\n"))))
 >+                         (chmod wrapper #o555))

Do (search-input-file inputs "bin/java") to avoid depending on input 
labels.  Also, you are using /bin/sh (which IIUC will be patched to 
/gnu/store/.../bin/sh), but for such patching to work, you need to have 
bash-minimal (or bash) in the inputs -- while bash-minimal is an 
implicit (native-)input of many build systems, it isn't a regular 
implicit input, at least for copy-build-system, so this wouldn't work
when cross-compiling, so I propose to add bash-minimal to the inputs.

 > +                       #t)))))

Trailing #t are obsolete nowadays.

(*) At least, I expect them to be; the download is in-progress.

Greetings,
Maxime.