If you were to hand the complete Onan to an experienced, competent engine rebuilder who would conduct a thorough rebuild, then you should budget two grand. Onan parts are expensive and if you give it to some shop that does a half-ass job, then you run the risk of it blowing up on you in the not too distant future. You can re-power the tractor with a brand new Vanguard but that won't save you very much money because you will need an electric clutch kit as well as the engine.
Here's the bottom line. A Case tractor with a dead engine is only worth about $200.00 to $300.00 tops. Seller's often ask for a lot more but the reason they are selling the tractor is because they don't want to dump nearly two grand into it. A running 448 with a deck will only bring $900.00 to $1200.00, depending upon condition and location. Do not enter into this deal unless you intend to keep this tractor for at least ten years. No one will pay you big bucks just because the motor is new or rebuilt.