Remove the drain screw from each carb in turn and examine the drainage. Is there hope?
Get some spray carb cleaner, and squirt into the drain hole. Stop when it drains out clear.
Apply vacuum (I use my mouth if not in rage of my mity vac) to the fuel petcock to allow the fresh gas you just put in the tank to flush out the carb bowl until it runs clear. Replace the bowl drain screw.
Do each of the remaining carbs in the same way. Don't forget to apply vacuum to the petcock to get all the carb bowls full of fuel.
Check the oil supply.
I'd probably pull the plugs to see if they are likely to fire.
You'll need a good battery. Apply full choke and crank it till is starts. Don't overheat the starter motor. 30 seconds on, 2 minutes off.
Cheers,