If it has been sitting for that long and you don't know the maintenance history of the ski before it sat that long then I would do the following:
Get rid of any gas in the system that has been sitting that long and replace with fresh gas. Clean the fuel filter and check the condition of the o-ring that seals it.
Check if it still has the gray factory fuel lines (marked Tempo). If so, then replace with automotive grade lines. Vent lines do not necessarily have to be changed.
You said you are going to go through the carbs so rebuild them according to the sticky thread at the top of this forum. Only use original Mikuni rebuild kits and parts. The fuel pump is part of the Mag carb.
The shop manual states to preform a pressure test if any components of the fuel system are disconnected and gives the procedure.
Also change the oil filter, check the oil pump is set up properly and check the condition of the 2 small oil lines that run from the oil pump to the engine. They should probably be replaced if they are old and brittle. I am not certain if the old oil needs to be drained and replaced from the tank and crankcase if it sat for that long.
