What Welder Dave suggests, but here is a suggestion that is really off the wall:
Also depends on where you are coming at the issue from. Since that loader is 45 years old, I'd be guessing that it is on a farm or somewhere where you don't much need to make a buck with it. For example, pretty much all my iron is in the same boat as you: I run a 51 year old Case 580B every day on my "stump ranch" feeding horses, getting firewood, clearing snow, and just about anything else you can do with a rubber tired tractor/loader/backhoe. My newest farm tractor is 1963. My d6 is 1973, D4D is 1969, etc.etc. Making do without using new OEM parts is the norm around here.
I'm doing a little guessing here, but isn't your machine drive from the engine to the transmission pretty much like most of the other small Caterpillar drives: A short drive shaft from the engine, which pretty well divorces the engine from whatever transmission is behind it?
So an option might be to find a suitable donor engine from something wildly different, such as a Ford 7.4 V8 diesel or the like. I'm not sure I would suggest a gasoline engine but some sort of diesel that this more popular and 'way less costly to acquire (than Cat engines) might be worth looking at. Something where parts are a little more common and not as pricey.
I suggested a V8 diesel because I'm not that familiar with any other diesel pickup engines--I'm sure there are lots of candidate engines out there.
It seems to me that the Cat engine mountings are basically 3 points: 1 point up front and 2 points back by the flywheel. So the connection to the transmission is either a short shaft with some rubber shear couplings which is the stock setup. Just use 2 U-joints which will not require careful alignment of the transmission to the engine.
The only significant challenge is to pick up a hydraulic pump drive location off the donor engine. The easiest location is off the front of the engine but that location depends on the clearances available.
YMMV!! Let us know how you are going about fixing it!
Jon.