I know its not exactly what you were asking about, but I thought I would weigh in on this "new engine design/old engine design" issue that seems to keep popping up.
The "old iron" - the onans, magnums, k series iron block singles, etc were definitely built to last, and often to be rebuilt and last longer still. It's amazing, when you look at how many of them are still chugging along after 30+ years of lower grade oil, lack of attention, misuse, etc! They were overbuilt, and expensive to build and rebuild. Add that many of these designs are no longer in production (parts are not readily available or very expensive due to limited production), they were gas hogs (compared to more modern designs), and they required regular maintenance and attention.
Like so many other areas, we have gone to easy production, limited life, throw away motors to replace these old engines. The intecs, vanguards, command series engines, both single and twin are lighter, more fuel efficient, lower maintenance, often smaller, BUT usually throw away after a shorter life span. They are cheaper to build/buy, but take less abuse and are more prone to be "unrepairable" when they do break.
I draw a correlation to the car engines of today vs the engines of the 60s and 70s. love the old motors - I could work on them, rebuild them, etc. BUT none could touch what you can buy in a modern design crate motor - a motor that makes more power on less fuel, and requires less regular maintenance!
Back on topic - I just rebuilt a magnum 18. I got lucky and it didn't need a lot, so the price wasn't too bad. But a full rebuild (with new parts) would have easily cost almost as much as a new twin! Now the new twin may not last as long as a fully rebuilt magnum, but, based on my use, either design will last longer than I will!
Some applications, I rebuild, nurture, restore the old school motor - because it looks right and I like the design, but I realize that it may not be the most logical decision. Other times, for a worker, I want the best I can get for the situation. Case in point, I am replacing the onan in my 316 Deere with a Kubota diesel - and will end up with a much better tool. When my Ford LGT
needs a rebuild, it will get a new design twin instead of the k341 in it now - more power, better fuel consumption, less shaking, etc. My round fender Deere 110 will ALWAYS have a k single in it though!
The new engines have been around long enough that there are good deals out there, as long as you can do the transplant yourself. But as always, your tractor, your decision. Just keep it going and have fun!!!