I'm gonna say that transaxle more than likely won't be of much use. What you got there looks and sounds alot like it's for a variable belt setup.
There's a thread at the top of the forum page for ID'ing transmissions and transaxles.
As for the aftermarket gears, it's primarily changing out the pieces made from powdered metal with factory replacement steel equivalents.
the kit you are referring to for the 820 is the 017 kit, which from what I had heard and read on here was it was discontinued and hard to find.