I now own two Apollo AV-1's and one Dacor SV-900. One of the Apollo's is much newer than the two other scooters and is super fast compared to the other two.
They usually slow down because the brushes in the motor get grease on them from the gearcase. It is relatively easy, but time consuming to open them up, remove the motor and clean the brushes and relubricate the gears. Also a good time to replace the rear seal and orings and check the prop seal and bearings. Unfortunately the cost to have it done is not cheap, it takes a few hours of reasonably skilled time to rebuild one. Parts are available but not inexpensive. If you can replace the water pump on your car it wouldn't seem too difficult