I would bite the bullet and get a new one.
Firstly, its new. What happens if you buy an expensive adapter... and the scanner dies?

2nd.. I don't know the specs of the UMAX but you may end up with a better machine.. better bit depth/color/resolution.
And the new one will have a warranty and you won't have to deal with the headache of trying to make it work.

If you want to get out as cheap as possible... I would hunt down another G4.