I'm a bit confused what VX bits do you have already? It's not so simple ideally you'd need a complete engine as a lot of internal bits differ, lower compression, stronger parts etc.. oh and bigger valves which I think are sodium filled on exhaust side. I think you'd be better to tune what you have new carb, cams and inlet manifold. As regards cam all Beta VX had the dizzy drive on the cam but I think Fiat RWD variations like Argenta VX and Pininfarina Europa Spider VX were different. For best power I believe most folks use a NA inlet cam (Fiat Ritmo/Strada 130tc or Beta 2.0ie) and a VX exhaust cam. A dcoe mounted on a custom inlet manifold direct to the blower seems the preferred high power choice with a standard inlet manifold you'd need a dcnf but due to the limitations of the manifold this will compromise ultimate power, to fit a dcoe you need a new radiator moved to one side this will release space in the engine bay at the slam panel area so that the carb will fit (sort of! it is still quite tight once you've sorted an air filter)