I think you might be onto something. If it does not cough and/or splutter, when it gets to 55 then it is probably restricted. 535’s are often restricted I think for the A2 License category.
Never seen it, and it can be done in different ways, but one I have heard of is two washers fitted inside the exhaust where it connects to the the cylinder to restrict it. If that is all it is, removing the washers should do the job or fitting the new exhaust.
Worth a try anyway, but if that does not do it you might need to look at the carbs. One common problem with 535 carbs are the diaphragms that start leaking air. They are easy to remove and check for holes(without remocing the carbs) and can be somewhat expensive to replace.
Let us know how you get on.