Some people have had good success with a X-ring or quad ring (same thing) like one illustrated here:
<http://catalog.precisionassoc.com/product/precision-associates-inc-x-ring/x-rings?>
Use one with the same dimensions as the stock O ring--don't know what that is off hand though.
Having your case machined for a proper oil seal is the long term answer and you can do the shift shaft seal at the same time.