Simplest way to do this would probably just be finding an Mk1 reactor design that uses a double uranium cell and then put a single plutonium cell instead, or a double plutonium cell in place of a quad uranium cell.
At least... I think Plutonium produces heat (and energy) twice as fast... You'll want a failsafe in place just in case it's even faster, but i'm pretty sure it's double the speed and double the duration, which means all you're essentially doing to the design is doubling its lifespan before the cell dies.