While the principles of internal combustion engines are straightforward, I honestly think that having a lot of experience with high-performance gasoline engines almost hinders the ability to work on these compact diesels. They are both extremely precisely built engines, but in fundamentally different ways.
For instance, it is not at all harmful to the tractor engine to run it at wide open throttle for 8 hours straight, without ever backing off the throttle. It's actually easier on the components. Running the engine at wide open throttle will only overheat or damage the machine if there is something drastically wrong with the cooling system.
If the tractor only has 700 hours on it, really the only way it could be worn out is to have been dramatically abused. If the rings have worn enough to drop compression to only 110 PSI, there was an air filter problem. Make sure you determine and correct that issue before you spend money on the engine. Similarly, the tractor should never have overheated, so there is a cooling issue. Spend money to have the radiator rodded out, and get the head and block machined flat and inspected for cracks. The rest will be a waste if that doesn't happen.
This vintage of Japanese compact diesel were designed to be industrial equipment of small size. They aren't heavy-duty consumer equipment or anything like that. People in Japan earned their livings with them in their specialized farming applications; consequently the block of your 7100's engine is equipped with wet sleeves, that can be pressed out and replaced ad infinitum.
Whereas in older dirtbike and quad engines the typical rebuild procedure is to bore the cylinder each time the engine is rebuilt, with these tractors the pistons can be simply re-ringed and run in the sleeves until the sleeves are out of specification, and then the sleeves can be replaced, potentially even re-re-re-using the original pistons. Unless the bores are severely out of round, there's no reason to bore the cylinders. And, if they ARE out of round, you're probably wiser, as others have recommended, to simply replace the sleeves entirely.
How did you check the compression? Remember that normal compression on your 7100 is 398-454 PSI, so a normal compression tester won't work. The minimum allowable according to Kubota is 341 PSI, so I'm suspicious of your numbers if the tractor runs at all. These machines have a compression release that can hang open or be out of adjustment, too. I'm not saying it isn't due to blowby, but if all the cylinders are at 25 to 50 % of their proper compression, it's also possible there is some major piston damage. This is often due to ether being used to coerce the machine to start.
I know the engine is apart now, so it's kind of too late, but with these types of engines, it really is better to spend the money up front and do it properly, because the engine will be good for thousands of hours and decades worth of operation very easily. If you're worried about removing the crankshaft being too much work after you've taken the front half of your tractor apart, you're worried about the wrong things, in my view.
Pull the crank out and check it for damage. See how out of round the bores are. Check the pistons for cracks or wear. Replace with stock specification components as needed. Take the radiator to a real radiator shop, and have them rod it out. Make certain the air filter assembly is in perfect condition. Re-assemble to specification and never worry about it again, just change the oil every so often.
Good luck; take pictures as you do it, and let us know how it comes out!
