You have probably already worked it out but I happen to have been searching for something and realized that Grizzly
www.grizzly.com carries woodlathe chucks right up to 6" with 1x8 threads. They should have something to fit the machine for under $100. It may not be the quality of a Multistar and if you want to do something with that chuck maybe you can contact these people:
http://www.jacquescoulombe.com/
Grizzly has chucks for simple operations like turning thin spindles all the way up to turning things like bowls and plates.