Compact scooters such as the Scout or Phoenix series of scooters from Drive Medical or the GoGo scooters from Pride Mobility cannot accept a different or larger seat. Most of these scooters have lower weight capacities and a larger seat would reduce that weight capacity by 15 lbs or more. Additionally, a taller seat such as a captains's style of seat would raise the scooter's center of gravity making it a tip-over risk.
Some scooters do have seating options as offered by the manufacturer; a wider seat, captain's style seat, etc. Those would be the only options you would want to consider.