If there was even a situation where one takes a certified plane and moves it into the 'experimental / prototype/ exibition catagory this is the poster child..... IMHO... About all you can do is to rent a truck / trailer and go get the complete project. Bring in back in your possesion and get it flying.... Since you are in the sky diving business using it for a jump plane is out of the question but at least you will regain your property. Take it to Oshkosh and display it... If there is an investor anywhere they will be there to inquire about the potential... The guy who has it now has absolutely no incentive to see the project through.
Sorry for your shitty deal.