Parts for that little Italian diesel (marketed by Briggs and Stratton back in the day) are close to unobtanium. However, with a new engine, you'd be good on hard parts for a long time as a show piece. Seals can be bought through the normal bearing houses... just give them dimensions, and they can get them for you. Diesels are a little bit different, but the previous advice is sound. Drain the fuel, flush out the fuel lines, and fill with fresh #2 with a healthy dose of fuel conditioner(Howes or Power Service). Doubtful the pump needs to be rebuilt, or the injectors. But, they may be gummed up, and that is where the fuel conditioner comes in. Get it started and run it for a while to clean things up. Jet A or #1 kerosene mixed at the same ratio as conditioner will do the same thing.
That's a rare piece, and I am sure the price will reflect that. If you want a working tractor, a JD 318 with 3point may be a better, and cheaper alternative. Likewise, many vintage and newer LGT's can be had with as much power and equipment availability for cheaper. The 920 in NOS condition would be a $5k tractor, if the wore out gas versions are fetching $3000 or better! To put it in perspective, I can buy 60 HP tractors all day for that, in working condition (5k that is).