This is as much a complaint about high prices as the fact that Briggs & Stratton keeps making and selling an engine with an absolutely awful design.
If you want the boring details: it is a single cylinder 4 stroke motor. The first issue is that while it is dead simple, it does not have a modern way to control the RPM. These motors are meant to run at roughly 3600 RPM but at full throttle (how they are meant to be operated once in use) they will easily exceed that especially with no load such as in neutral. So they have a mechanical governor, which is also combined with an oil slinger (exactly what it sounds like: it is just a thing that slings oil all over the place hoping to get it onto the parts that need it). This contraption looks like this: https://www.lawnmowerpros.com/prodimages/691968.jpg. It is not very sturdy and is not very well mounted. It tends to break or even jam the gears between the crankshaft and the camshaft. This is bad because you don’t want loose pieces flying inside the bottom end of the motor.
In addition these motors have puny starters, so they have what’s called a compression release, which is a weight and a spring on the cam shaft. You can see it right by the gear here: https://www.briggsbits.co.uk/acatalog/84005207.3.jpeg
The idea is that at low RPM such as while you start the engine, the tiny spring will hold the weight to the shaft and the bump on it will keep the exhaust valve slightly open. This will mean you don’t get normal combustion but also it will be a lot easier to turn the engine over which is important for the puny starter. At higher RPM the weight will fly out and the other side with the bump will retract causing the exhaust valve to close fully when necessary.
The issue with this design is that the spring as you can see is tiny and thin and the weight is also pretty flimsy. They tend to break off and then you have a chunk of metal again.
Speaking of the camshaft, notice that the cam lobes are pressed onto the camshaft, not cut out of a single piece of steel. This is cheap but tends to be very sloppy. These motors don’t run very well.
And lastly, while these motors have a (tiny) oil pump and it is properly driven by the camshaft, the part that drives the oil pump is actually a separate piece of shaft that just slots into the main shaft of the camshaft. You can see it here as the short shaft with two flat parts on either end: https://www.lsengineers.co.uk/media/catalog/product/cache/21...
This means the camshaft isn’t even straight necessarily but is gear driven which requires more precision than chain, not less.
This is before we get into just basics like the fact that none of them have proper head/cylinder mating surfaces so they blow head gaskets like crazy, or their very loose rockers that go out of spec all the time causing it to run very poorly and needing constant adjustment. This is the same exact engine in every smaller riding mower on the market today. If you don’t get this one, you will get a much worse Kohler equivalent.
An equivalent Honda engine I could swap into my mower would cost me $2,000 for a refurb. Harbor Freight used to sell a motor for this for $800 but discontented it a few years ago. Basically, the entire market segment is shit. Greenworks and Ryobi have the electric mowers for about $5,000 that don’t seem to have these issues but batteries don’t have nearly the energy density than the two gallon of gas which is what it takes to run a normal mower for an hour.
