Oga how much to replace engine for a 2002 corolla Le?

Hello sir,
I recently had issues with my car transmission. It threw codes po 780, po 775 and po 750 for the transmission and i think...I was advised to change transmission which i did. A few hours after leaving the mechanic's place, the D light was blinking when i parked...by the next day, it was blinking while driving and slipping out of gear...i have since parked the car. I filled the new transmission with atf z1 when it was installed. mechanic is saying brain box needs to be replaced now...what do you advise?