Cams are different profile, but not radacally. I had an MFI cam in my old Mk2 xr2 (was just a spare one had lying round) and did not notice any difference i nthe way it ran or idled. I woul dbe checking your cam timing because the belt may have jumped, then re check ignition timing and get a strobe light on it to be sure. If you wanna find TDC of number one take plug out and pop a plactic drinking straw down the plug hole and turn engine - piston will move straw up and down to indicate tdc of the bottom end.