The gear boxes are physically the same, but have different gear ratios.
You cannot use a tiller gear box to replace a
thrower gear box and vise versa
Been a few years since I worked on them, but seems like one is a 2 To 1 ratio and the other is 1.5 to 1 or similar ratios, IIRR
No new gears, are available but they show up on the internet sites and our sponsors at the top of the page.
I have both but they are not for sale.