Luke on the front of the engine you have a fast idle adjustment knob (as pic below bottom right of picture, this is from a 6hp but similar design), where is this set to ? Is it fully in (anti-clockwise) or have you tried turning it out a little to start ? Try turning it clockwise a bit and see if that makes a difference
How well do you know the service guy ? Is your motor now pumping constantly from starting, if it is I suspect he's simply removed the thermostat, rather than actually changing the impeller etc. You may not even needed that doing. Are you aware the model you have only pumps when it needs to ? So its not uncommon for it to start and stop pumping water when running.
If he has removed the stat it could be running cooler or even too cold which has affected the idle, hence why its cutting out now when engaging gears. Just a thought
Peter @ Boatsandoutboards4sale ~ www.BoatsandOutboards4Sale.co.uk
~ 07930 421007