The bore and stroke of the Crossfire is 89.9x84mm. The SLK350 is 92.9x86mm. It sounds like they probably just bored and stroked the existing block, so the head might bolt up. However, due to the different head design, I don't think you would be able to use the Crossfire intake (and maybe not the exhaust). You would have to use the SLK350 intake, fuel injection, ignition, and therefore engine electronics. Regargless of whether the intake would work or not, you'd need different engine electronics to use the variable valve timing. The timing chain system would also have to be changed since there is an extra cam on each head. It would probably be more cost effective to do a complete engine swap once a few are available as salvage.