It could be prop wash. My Classic 35 always veers to starboard under power and will eventually do a full circles without some constant rudder pressure to port.
I think the test is whether it still veers to starboard under sail.
Is it a case of the helm not being centred? Maybe some adjustment on the cables is needed?
You didn't mention why it was necessary to take the rudder apart. Did you witness the rudder being re-assembled and ruled out the rudder stock being slightly bent?