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The internal gears in the jet drive unit should outlast several powerheads if the oil is changed regularly and the the seals are good so that no water gets in. The parts that wear out are the impeller and the appropriately named wear ring. Those can be replaced anytime without removing the engine. Unless you suspect some problem with the gearbox, I wouldn't replace the drive unit. Just check out the impeller and wear ring and replace them if necessary. Do check the condition of the input shaft seal just below the motor. If that looks questionable (corroded or stiff), replace it while the motor is out.

Sportjet engines don't have water pumps. They get cooling water from the jet pump. About the only thing that can go wrong with the cooling system is the thermostat sticks or sand gets in and plugs the internal cooling water passages inside the engine. I found a lot of sand in the nooks and crannies of the cooling passages in my engine when I rebuilt it.

Two stroke engines can be damaged by running too lean. Temps in the combustion chamber of a lean running engine can get hot enough to melt pistons. Keeping your carbs adjusted is important to the longevity of the engine. Reading the color of the sparkplugs is a common way to determine if the mixture is correct. Carbs aren't too hard to clean or rebuild when they are on the bench. I would at least take the float bowls off and blow out the jets with compressed air if I had the carbs out of the boat. There are several different sizes of main jet available for these carbs. If you are running a boat set up for higher altitudes (smaller main jet) at sea level, it could be running too lean. The size of the main jet is stamped on the jet itself. The jet is a little brass piece that screws into the bottom of the carb. You can see it when you take the float bowl off. Before you do any of your own work, I would recommend buying a manual. The Clymers one is not bad. In some ways it is better than the factory manual.

A properly tuned and maintained 2-stroke power head should run several hundred hours, which can be many years of use depending on how much you boat. Many of them die prematurely from improper lubrication, running stale gas, corrosion during storage, and other kinds of neglect / abuse.