Did you get/see any service history on the motor? private sale or a dealer?
The lack of acceleration/sluggish operation sounds like a bit of a fuel issue. I can "suspect" that the previous owner decided to fiddle with the carb adjustments (to keep it idling without stalling) which has resulted in the very fast idle, rather than pull the carb and clean it out properly.
This isn't "grand-dad's" 2 stroke. A 4 stroke requires (did I say REQUIRES) annual maintenance whether it's used every weekend or only once or twice.
Too many owners bought the 4 strokes on all the hype about how much better they were than 2 strokes, but most dealers failed to adequately instill the fact that the yearly maintenance in the owners manual must be followed to the letter or you end up with a heap of steel and scrap aluminum in fairly short order.
If you don't have a fist full of service records to go with this motor, and you want it to last for you (beyond this season), you are going to need to catch up on 13 years of missed maintenance.
The carb needs to be pulled, cleaned out, and rebuilt if necessary - at minimum, new gaskets are in order. Change the sparkplugs, change the engine oil, inspect the water pump and unless you KNOW when it was changed, a new impeller is in order. Change the gear oil in the lower unit. Grease everything that has a grease nipple and anything else that looks like it could use it.
If that sounds like "alot" or "excessive" that is really just "typical" prep for a used 4 stroke.
After all that is done I would run her for the season. If she seems to hitch or hesitate a bit or picks up slow, at the end of the season get the valves inspected and adjusted (should have been done within the first couple of years of operation and then checked every 3 to 5 years there after).
Four strokes don't like to be stored, they want to run all the time which makes them "maintenance intensive" in marine applications...
