Originally Posted by TimJ
Scroll down to find the cribsheet on cassette/freewheel spacing. If you measure and find out you have an "ultra" freewheel then you're in luck. I believe you would be able to use any Suntour 6 speed indexing, or possibly even 6 speed non-indexing (provided it was designed for the ultra freewheel) derailleur and it would index with your current shifter. If you have the "accushift" freewheel then only a 6 speed accushift rear derailleur will work.
Oh, I entirely forgot about the freewheel issue. Actually, I never had the original freewheel. I use a 5-speed Shimano freewheel, and indexing is fine - maybe the Alpha-2000 was even intended to be used with regular freewheels since it was bottom-of-the-line back in 1989.