First off, the "aluminum" bore engines use a lining called nickelside. It is extremely hard, so hard they cannot be bored out. They are just as durable as the Cast Iron if not more so, but they are disposable rather than rebuildable as the cast iron is. The nickleside wears less than a cast but does not resist warping as well during abrupt temp changes such as hosing off a hot engine with water. A nickleside engine has a mean time before failure of 1k hours. An intek or courage cast engine has a 500 hr mtbf. NS engines nearly always outlast the equipment they are used on. They are used almost exclusively on snow Blowers, lawn mowers, wood splitters pumps, generators etc. They are extremely durable, They will outlast a cast engine almost every time if maintained remotely. That post you saw on the 3.5 Briggs attempt to kill was a NS engine. If that had been a much softer cast liner, it would have died much sooner. One of the reasons they are used on snow blowers is they hold up better in the cold weather starts with limited oil supply, lighter weight, 40 years proven in the field as outlasting the cast iron machines. So, dont fret, that engine will be running 20 years down the road as well as it does now as long as you change the oil once in while.
Now, to the gentleman stating "buy a cast iron engine from China" You sir, are sadly mis-lead. those are true throw aways and will not hold up like a vanguard or a command under anything remotely close to commercial duty. How many have you seen on a Zero turn Exmark? skid Steer? Garden Tractor? Cuts? The commercial guys are coming back to the Briggs and Kohler in droves as the Honda's do not hold up any better and are very hard to get parts for and double the price and you wont see many robins on an Exmark. Its like trying to put a 9 ft plow on a "one ton" Toyota and start plowing commercially with it...how long do you think it would hold up with those wimpy 1 inch drive shafts and frames that are 1/4 the size of a ford super duty. How many have you seen out there working commercially? hmmm? there a toy in comparison. sure there fine for they people who never work them, but for the people that do actually "use" a truck, never. Not to mention the lack of patriotism. Sad.
I am 46 yrs old and retired. I have been working on, using, abusing OPE since i was 12. My first tractor was a 1971 Massey Ferguson MF-7 and i still have it...never had a wrench on the motor and still runs as new and it has no cast iron sleeve...i have an old western auto push mower that i use as a brush hog along the sides of my 800 ft driveway, the deck is trashed, one wheel came off as the chassis failed, bunch of brazing patches but guess what, that old Briggs 3 hp is still going strong. I now have a collection of 30 or so tractors and a bunch of snow blowers, the oldest being around a 1970 Toro 726...guess what, no cast iron and runs as new, starts second pull every time and uses no oil. In all the time i have been tinkering and collecting these things, the last thing i worry about is the engine. I worry much more about the previous owner than the name on the engine. Since the boom of the Internet forums, we here about failures...just think for a moment how many lawn mowers and tractors are out there vs the number of failures, many caused by the owner, out there. what, maybe .001%? Truly, engine failures are extremely rare no mater the engine. There have been some bad ones...Kohler KT17 comes to mind, but even then the failure rate was less than 3 percent and most are still running fine. The failures were caused by poor side hill lubrication on 15 degree or steeper left/right angles. If you ran them up and down or didn't exceed 15 degrees side, which is really pushing it, you'll never have an issue. Most failures came were the engine was mounted with the cylinders facing side to side, in that case, climbing a 15% grade would starve the engine of oil. I believe the JD 317 had that issue and highest failure rates. I could ramble on here forever providing examples, factory mtbf ratings but the fact is, no matter the engine, other than a Chinese one working commercially, engine failures are very very rare. It is the last thing to worry about as there are many other components that fail far more commonly than an engine on a OPE.