Matt,
Congratulations on getting the tractor.
Parts are not a problem. Contact Brian Hildreth of Salem Power. He is in Michigan but is happy to ship parts to Canadians. There are no Canadian dealers to speak of so I would not bother trying to find any. Brian is one of the very best dealers out there. He is very knowledgeable, extremely helpful and his prices are never inflated above MSRP. Talk to Brian about the banana plate replacement kit he has. This will restore your foot pedal operation back to the way it was when the tractor was new.
Here is the URL to his website......
Contact Us
Now let's talk about the engine. Truthfully speaking, you would be wise to rebuild the Onan if you intend to keep this machine for a long time and use it a lot. However, Onan parts are not inexpensive to buy and it can cost as much as $1800.00 for a professional shop to conduct a thorough rebuild. Case selected the Onan CCKA engines because in 1972, they were the best that money could buy. The fact that your tractor, which must be at least 34 years old, and still has the original engine is a testament to the durability. The 646 production ended in 1980. You mentioned diesel and those too are long-life engines when properly cared for. If you rebuild the Onan, then the rebuilt engine will drop back in with no issues because it is 100 percent compatible with the tractor.
If you switch to a diesel, you now have many issues to deal with. A water-cooled diesel will need a radiator. It will also need a return line to a fuel tank that has no provision for a return line. There will be wiring issues and throttle control problems. Adapting the hydraulic pump to the diesel's crank stub is another item on the list. A suitable exhaust system will have to be worked out and of course, the diesel must be mounted securely in the proper spot so it does not interfere with anything near by but is also on the correct plane to line up with the hydraulic pump.
If you have your heart set on replacing the Onan, then go to Princess Auto and take a look at the 10 HP air-cooled diesel they sell and sometimes put on sale. Keep in mind that those engines are actually a clone of the Yanmar diesel but are made in China. That should not be a concern because Princess is there to back it up should something go wrong.
Do not let the fact that these engines are rated at only 10 HP trouble you. Several people have installed them in Case 444 garden tractors and are very happy with them. There is more than enough HP and torque in that diesel to look after the needs of your loader/tractor. The Case hydraulic drive system cannot put more than 10 to 12 HP to the rear wheels no matter how much engine HP is driving the pump. That is not speculation. It is simple fact that any hydraulics engineer can confirm mathematically. You are not going to install a mower deck on this tractor so extra HP will not be realized in the hydraulic system.
This engine comes with its own exhaust system and fuel tank. It is a simple conversion and the hydraulics guy at Princess can help you with the pump adapter and any Lovejoy coupler issues that may crop up. You can go to the local auto wreckers and find a suitable 12 volt cooling fan from the radiator of a car and that will keep lots of air passing over the oil cooler. Just wire it into the key switch terminal that used to supply power to the ignition coil on the Onan.
Yes, you will have to open the hood to refuel this engine but so what? These engines are very frugal and you will get several hours of use from a single tank of fuel.
Here is the URL for that engine.
10 HP 418cc Horizontal Diesel Engine with Electric Start | Princess Auto
Of course, you could also re-power the tractor with a Honda, Kawasaki, Vanguard or Kohler twin with ratings from 14 HP to 27 HP if you wish. Places such as Small Engine Warehouse sell surplus engines for reasonable prices but many do not come with exhaust systems. If you do not get an engine with the correct diameter crank stub or crank stub STYLE, then adapting it to your pump can be a problem. The Princess Auto diesel is the most economical solution to your problem but that is just my opinion.
If you do decide to do an engine swap, then you would be wise to do the following. Remove the Onan carefully. Remove the spark plugs and pour a few ounces of motor oil inside each cylinder. Install the spark plugs and then with a pair of visegrips clamped to the crankshaft, rotate the crank back and forth to spread the oil onto the cylinder walls. If one cylinder is coming up on the compression stroke, then the oil will stop you from turning the crank further, so do not force it.
Keep every single part that you remove from the tractor as a result of the swap and put them into a box. Store the box and the engine some where out of your way, preferably in a spot with a stable environment. If you ever decide to sell the tractor, it will be worth a lot more to a collector if you have all that stuff. He will remove the engine you put in and restore the tractor back to what it was originally.