A dealer telling you to buy an aftermarket carb? Thats an odd one.
Depends which carb is fitted to your machine, there is one with an accelerator pump that can give problems.
Most recommends swap is to change out the Walbro 199 for a Zama C3EL42. Some people like and can repair/refurbish the Walbro's and claim they deliver more of a top end- others say the Zama's cure any and all the Walbro's problems and the average user does not notice any loos through fitting one- so you decide for yourself, but it could be your own new to you 359 runs just fine.
If you do decide to swap carbs- I am willing to bet someone tinkering with a 346 build will buy your old carb off of you.