There was a kit made by a company called Paltec. It included 2 sprockets and a chain that replaced 2 gears inside the hi-low gear box. The one in the thread below is now in the hands of another member. He looked into having them reproduced at local machine shop at a cost of $500 - $1000 set up fee and then another $150 per gear sprocket set. Good luck finding one! I've been looking for the same reason.
http://www.3wheelerworld.com/showthr...ec+reverse+kit