Problem with gearbox

[SPYROS29]

Hello to the team,

I would like to post you my problem seeking for some answers from your valuable experience at classic Audis!
I have an Audi 80 typ 81 and last year I converted from 1.3 (EP engine) into 1.8 (DR engine). Also I changed the gearbox from the 4 gears 6V to the 5 gears ET.
My problem is this: The car runs fast with the first 3 gears and then falls gradually. I don't have the same torque on the 4th and 5th gears. Especially the 5th is like not exist. That means that I barely can achieve 150km/h.
I suspect that the problem might lies to the flywheel or the camshaft. What is your opinion?
I know for sure that the mechanic changed the flywheel during the conversion but I dont know what he put instead. Any suggestions?
I appreciate any proposal to solve the problem.