The fitting you are talking about is the pulse line for the fuel pump. I run a Dellorto and go straight off the Mikuni round pump to the inlet. Other carbs may vary.
Water wetter is fine.
For the intake boot and clutch part here’s the secret. Search ebay for ATK 250 as this engine was used on ATK dirtbikes here in the states. I know there is a boot and clutch cover there right now. As they have been listed for some time you might want to deal with the them for a better price. It worked for me on the couple items I needed for a spare motor.
These are strong little motors and last well when kept a little rich. Shift gently and take care of the gearbox as gears are expensive and at times hard to find.