Re: 1998 40 hp force
First check would be compression, there are others in this forum who own force outboards and they will give you a better idea of them.
Does it have a history of service, does it start first kick and pump plenty of water, idle smoothly, do you know how many hours are on it?, all questions you would want to know the answers to. As I say though others here will know more about the actual motor.
A 98 model is now 16 years old, so you would have to expect some ware and tear. Being on a pontoon in fresh water I would expect the motor to be in reasonably good condition, depending if they lifted the motor out of the water and regularly maintained it.