I suppose it depends how original, you want it to be, but a rebuild should allow the engine to run unleaded, higher compression, maybe slight cam, rollers etc, for a smoother more powerfull donk without guzling the gas. I know Ray will recommend a crate engine, his is a great changover, but I'm not sure what else has to be done with mating to box, or mounts or or or or or .... But one thing for sure, as my old school song used to say " Now is the time......."