Take it slow turbo, do some more searching and reading, this, for example: https://www.mopedarmy.com/forums/read.php?7,2753326,3680039#msg-3680039 . Most importantly, like saab said, assemble it carefully with a good base gasket and get it running well so you have a baseline.
There's a guide in the wiki, with pictures, on how to enlarge the intake, but you would have to split the cases again for that.
How much you can safely take off the head depends on your castings/machining and what gaskets you use. You'll need to figure out what your squish (piston to head clearance) is, and what is a safe minimum.