If a cancer cell from outside gets into you and that cell has no mhc on it then how will the persons body recognise that its foreign and should be killed ?

There has been no instance of cancer spread in the fashion you conjure up. You can reduce your risk of cancer by adopting healthy habits: Eat a mostly vegetarian diet, avoid alcohol, tobacco and street drugs, avoid excessive exposure to sun, get HPV vaccine, exercise 30 minutes each day, drink enough water so your urine is mostly colorless.

Not having MHC would not deter the immune system. The Tcells work on a fail safe system where the correct MHC or surface molecule is needed to tell the Tcells to stand down. Hence, some cancer cells are able to avoid the immune system by presenting proper MHC and specifically, PDL-1.