What level would he be up to: SS1, SS2, SS3, SS4 or SS5?
For the sake of argument, let's say SS5 (the most powerful). If he were to revert back to the old Saiyan ways, who among the cast would be able to protect the earth? I think Trunks and Goten when they Fuse. It would take a long battle, but eventually Goku would fall to young ingenuity.