Can you confirm the exact model you're working on and manufacturing date: ie:
FJ80L-GNPNEA ?
Original Torque convertor part number AFAIK should be: 32000-60031 but is discontinued.
How many miles on your transmission and were you having any issues with it?
Has the transmission, TC, or engine been removed/replaced previously?
Can you post up a few photos of the TC showing any/all markings (numbers/letters/colored marks and the welds, original welds will look clean) and a close up of the broken part?
The TC in the link below may fit if it's still available. You could contact that company and ask about their process, they have
a good reputation apparently:
We supply the best torque converters and transmission parts money can buy. With over 35 years in the industry, we have over 1 million converters in stock!
gopnh.com
Information for the Toyota A440F transmission torque converter. We are the USA’s largest torque converter manufacturer!
gopnh.com
A good condition low mileage original TC out of a known running/properly shifting FJ80 may be another option.
Your mechanic needs to check the rear crank for a wear groove ie: if there's a deep wear groove a new seal may not seal well.