Just take a popsicle stick,small dowel,bamboo skewer,anything that will fit down the spark plug hole.Turn the engine over until the stick doesnt move up anymore.Then rock the crank to the left and right and when it doeesnt move anymore your at tdc.(just eyeball it).Youll never get true TDC because the piston doesnt move 2-3 degrees as its rocking or moving to each side of center.Youll be close enough.