Ok, i´ll contact KB to check if they can supply a single piston that fits. Gonna wait til monday if this dumb machinist will give me a feedback or not - no chance that i will give engine parts to him again. It seems very hard to find a shop you can trust over here.
Thank you guys for your input, positive or negative - i´m more relaxed now. If it´s not possible to get another piston i´ll run it as it is.
Michael