Where to get a TT2421-1 allison transmission rebuilt

I have a ford A66 with an Allison 2421-1 Allison Transmisssion and torque convertor It is weak.. cant hardly move itself after the oil gets warmed up It.was a bear to take out. No one seems to know anything about them even though they were used in a lot of John Deere and case wheel loaders. any body know where i can exchange one or have mine worked on ? Thanks John