Re: Pressure tank
I've never done this to a 10hp model and don't remember the layout or the motor, but if it has a bypass cover, there is no reason to replace it with a plate. In fact, the bypass cover may have a curved surface inside, so I wouldn't want to replace it, if that is the case.
Take the cover off, so that no drill shavings end up inside of the motor. If there are mounting bosses for the stock pump, my choice would be to use them. Those bosses would be drilled part way through and tapped with the correct threads. The center hole would then be drilled all the way through. Replace the cover, using a new gasket. Mount the pump, hook the fuel lines up and that part of the work is done.
Don't forget, however, that you now have a nipple under the carb that has no line running from it to a three "prong" fuel connector. Left that way, you will end up fuel/air mixture spraying into the cowl on each "backstroke" of the pistons. To fix this, remove the nipple and replace it with a flathead or panhead screw of the correct size, thread count and reach. Notice the emphasis on reach. If you put a screw in that is too long, it will hit the leaf plates, so be careful of this issue. You may also need to put a crush washer under the head of the screw, in order to get a good seal.
If you simply prefer the Mikuni, you can certainly go that route. You would avoid having to drill/tap the mounting bosses, but would have to take other steps to mount the pump somewhere else. From that point, yes, you could just install a nipple in the center boss of the bypass cover.
???
