Bike sure looks great! You didn't say which
gasket is leaking, some are easily replaced. Now before you go yanking the motor... I would start with the basics.
1. Go to SOHC4.net http://forums.sohc4.net/
and down load the factory service manual. Sorry, I don't have my CB350four anymore but that is where I found a link to the manual.
2. The petcock on that bike has a screen in it that filters the fuel and you can remove and clean it.
3. On the carbs, pull the four float bowl drain screws and see what comes out. If theres any crud along with the fuel, often you can open the petcock and flush them with the sdrains out. But maybe you'll have pull the float bowls and clean them well. lastly a carb O/H may be in order...
4. Do a good tune-up. New plugs, clean and gap the points, check the timing, clean the air filter, change the oil and filter. Also adjust the cam chain tensioner... all per the manual.
THEN if you still feel the need to go in further, have at it!