I charge out 4 hrs + I keep the old engine for spares if I am selling you the engine
Very rarely it actually happens in 4 hours because some thing always goes wrong .
Parts don't line up, wires too short, rusted bolt breaks off , threads different , air filters different , etc etc etc .
The scheduled service fee books list 2 hours but this is for swapping engines in brand new mowers with all the needed factory tools at hand .
OTOH I can spend near an hour just cleaning the crud off the mower before I put a spannar on it .
If you provide your own engine & want the old one back then I charge 6 hours as it will probably take all day to do the job and I have no retail mark up on the new engine .
If you have some lifting gear I would suggest doing it yourself and allow the entire weekend to do it
The only really difficult bits are removing the PTO pulley & manhandeling the engine.
The madening bit is locating the muffler
I have been know to fit 2 single cylinder mufflers on some twins because there is no factory muffler that will fir or the factory muffler is 1/2 the price of the new engine .
The most important tool in your tool box for a job like this is your camera
A before removal & after removal of every part, particulalry the controls