On most of my builds, I have used a new motor plate drilled for the motor I am using. In your case you would need to drill new slots or holes. Just be thoughtful of where you are drilling. Be sure your new holes will not weaken the plate. You said it's about 1/2 inch off? Using 5/16 bolts, that won't leave much material between the original holes and the new ones. This may cause weakness in that area ( can't tell without pics ). If the frame is not painted, you may consider welding a new plate in that is drilled for your motor. Just drilling holes all over the plate for different engines is usually why I replace the ones on my builds.