Pistons are easy to get. Wiseco, J&E, Ross, Venolia... You don't necessarily need a 'domed' piston to get higher compression. If the stock piston has a recess in it, merely eliminating that will normally give you a nice increase in compression. Regarding, the base gasket -- it appears to be slightly thicker than the DRZ "E" gasket, so changing to one of those will help. But, I don't think that will get you to the E's 12.2:1 ratio (Z400 ratio is 11.3:1). Another option is to machine the bottom of the cylinder barrel. According to some calculations I did a while back, lowering the barrel in the block .025" (by using a thinner base gasket and/or machining the barrel) will raise the compression ratio to 12.5:1. However, when you make those changes, there is a danger of the piston hitting the cylinder head, so piston-to-deck height -- the distance from the top of the piston to the top of the barrel -- must be respected in your modifications.
I'll check with a DRZ racer I know and find out what the deck height is.