A bit rusty, tried using $E=mc^2$ to figure out how much kinetic energy would 1kg of mass convert to, and then work backwards to figure out what would be the final velocity of a 1kg mass having kinetic energy equivalent to 1kg mass as kinetic energy.
Not sure how correct this is but here it goes: $E = 1kg\times c^2$ is the (kinetic) energy equivalent of 1kg mass. I am hazy about the next step, 1kg mass having kinetic energy of $1kg\times c^2$ w.r.t. to a stationary point would appear to be moving at what (relativistic) speed? (not sure even if the first step is correct)