Let me try to help you OM with my limited knowledge which I have gained after building my homebrew ubitx based on V5. Disclaimer: I don't own any version of ready made build ubitx :)
My first question will be where you have sourced your replacement IRF510. If it's from reputed source like Digikey or mouser then you can start the troubleshooting or else check if those IRF510 are genuine one.
Since you have replaced both the K1 relay and the MOSFETS at the same time. Check if there is any contact issue with the replaced K1 relay. Since your receiver is working, are you still able to listen to the receiver when to press the CW key ?
As the receiver is working fine I would focus on things located beyond C80.
73
Anthony
VU3JVX
