I don't think it needs a complete rebuild, just pistons rings valve. But anyway, I guess its cheaper to buy a brand new motor....
Let's say that you decide to re-power the tractor instead of rebuilding the Onan.
You will have to choose between a Kohler, Honda, Kawasaki, Generac or Vanguard engine.
The physical size of those V-twins will dictate how much you will be required to modify the hood of your tractor. The sides of Ingersoll models with Vanguards have bulges for clearance. Some engines required the hood hinge to be raised up.
The wiring of your tractor will be different than what is needed for the new engine and you may have to change your ignition switch.
The main issue will be the PTO clutch. Your mechanical PTO EZ clutch won't work on the new tractors unless you pay a machine shop to create a special main shaft to bolt onto the flywheel of the new engine. Therefore, you will be forced to buy an electric clutch. Expect to pay around $300.00 for that plus you still need some sort of adaptor to mount it to the flywheel. Oh...you will need to drill a hole in your dashboard for the switch to operate the clutch. Of course, you will have to buy that switch and a separate fuse as well as add the wiring to hook it up.
Moving now to the back side of the engine, you will have to find a motor with the correct size of crank stub. You may have to cut that stub down because they are usually longer than what is needed for a Case tractor. You will also need a mounting bracket for the hydraulic pump and you have to make sure that the engine you buy had threaded holes to accept a pump mount. The engine must be positioned on the frame of the tractor correctly so that the PTO pulley lines up with the mule pulleys for the snow blower and mowing deck. You must make sure that the engine is not mounted too high or too low or the stock belt lengths won't work for driving the deck or blower. Most likely, you will have to design and fabricate a mounting plate for this new engine because the current holes in the frame won't line up with the new engine's mounting holes.
The other stuff such as redoing the fuel line, choke cable, throttle cable etc will be a piece of cake compared to creating a new, custom exhaust system. You will need a new muffler, exhaust flanges, pipe and a bunch of pipe bends. A TIG welder is the most ideal method to weld up the many joins between pipe bends and straight pieces. With some luck, you should be able to turn out a nice looking exhaust system after a few days of work.
On the other side of the coin, you could remove the Onan engine all by yourself. If you have some experience with engines, you could dismantle it. If you don't have that experience, then you could just put the engine in your trunk, take it to a local engine shop and leave it with them. They will dismantle it, measure the cylinder bores to determine wear, taper and whether the bore is oval or round. They will measure the crankshaft journals to see if the crank needs to be turned. The valves and seats will be inspected and the springs will be tested. At that point, they will be able to tell you if the block needs to be bored and by how much. They will advise if you need new valves, seats, guides and springs. The camshaft can be profiled to make sure you are getting all the lift you should be getting. The head and the top of the cylinders may need to be re-surfaced if they are not dead flat.
Once they do all the necessary work and reassemble the engine, all you have to do is to put it in your trunk, bring it home and put it back in your tractor. It will bolt right back in with no measuring, no fabrication, no modifications. Everything will fit. Everything will work and you will have an engine that should be OK for another 20 years of use if you take care of it.
Unless you intend to keep this tractor for AT LEAST 10 years, do not do either one of the above. Sell the tractor as it is and go buy something newer with lower hours.